Humanitas is a Latin noun meaning human nature, civilisation and kindness.
Classical origins of term 
The word humanitas was used by Cicero to describe the formation of an ideal speaker (orator) who he believed should be educated to possess a collection of virtues of character suitable for an active life of public service; these would include a fund of learning acquired from the study of bonae litterae ("good letters", i.e., classical literature, especially poetry), which would also be a source of continuing cultivation and pleasure in leisure and retirement, youth and old age, and good and bad fortune.
Pliny the Younger defined humanitas as the capacity to win the affections of lesser folk without impinging on greater (Ep. IX, 5)
Revival in Early Italian Renaissance 
The concept was of great importance during the re-discovery of Classical Antiquity during the age of the Renaissance by the Italian umanisti, beginning with the illustrious Italian poet Petrarch, who revived Cicero's injunction to cultivate the humanities, understood during the Renaissance as: grammar, rhetoric, poetry, history, and moral philosophy.
In 1333, in Liège, Belgium, Petrarch had found and copied out in his own hand a manuscript of Cicero's speech, Pro Archia, which contained a famous passage in defense of poetry and litterae (letters):
Haec studia adolescentiam alunt, senectutem oblectant, secundas res ornant, adversis perfugium ac solacium praebent, delectant domi, non impediunt foris, pernoctant nobiscum, peregrinantur, rusticantur. (Translation: "These studies sustain youth and entertain old age, they enhance prosperity, and offer a refuge and solace in adversity; they delight us when we are at home without hindering us in the wider world, and are with us at night, when we travel and when we visit the countryside").
Petrarch referred to this quotation often, and where Cicero used the phrase "litterarum lumen", "the light of literature", Petrarch in the margin wrote lumen litterarum alongside and drew a sketch of a lamp or candle. The Liège manuscript is lost and so is Petrarch's copy, but Petrarch's copy "can be shown to be behind all but one of the later manuscripts" and preserve Petrarch's marginal annotations. Petrarch, in many respects a Medieval man, regretted that Cicero had not been a Christian and believed that he certainly would have been one had he not died before the birth of Jesus. Cicero's humanitas was not seen as in conflict with Christianity or a Christian education by the umanisti who immediately followed him.
Humanitas during the French Enlightenment 
The ideal of humanitas was first brought to Rome by the philosophic circle around Scipio and further developed by Cicero. For Cicero, humanitas was a style of thought, not a formal doctrine. It asserted man’s importance as a cultivated being, in control of his moral universe. The man who practiced humanitas was confident of his worth, courteous to others, decent in his social conduct, and active in his political role. He was a man, moreover, who faced life with courageous skepticism: he knows that the consolations of popular religion are for more credulous beings than himself, that life is uncertain, and that sturdy pessimism is superior to self-deceptive optimism. Man becomes man as he refines himself; he even becomes godlike: “Deus est mortali iuvare mortalem,” wrote Pliny, translating a Greek Stoic, “To help man is man’s true God.” Finally, the man who practiced humanitas cultivated his aesthetic sensibilities as he listened to his reason: "Cum musis,” wrote Cicero, “id est, cum humanitate et doctrina habere commercium". Virtue, Cicero insisted, is nothing but nature perfected and developed to its highest point, and there is therefore a resemblance between man and God: "Est autem virtus nihil aliud quam in se perfecta et ad summum perducta natura; est igitur homini cum deo similitudio".
Cicero’s humanitas . . . reappeared in the first century in Seneca’s claim – made in the midst of a lament over Roman bestiality – that man is a sacred thing to man: “homo res sacra homini”; and reappeared once more in the eighteenth century in Kant’s call for human autonomy and in Voltaire’s stern injunction: “Remember your dignity as a man.” In the beginning of his Meditations, the Emperor Marcus Aurelius elaborated a veritable catalog of qualities which, all together, made up the virtues which Cicero had called humanitas and which the philosophes hoped they possessed in good measure: modesty, self-control, manliness, beneficence, practicality, generosity, rationality, tolerance, and obedience to the dictates of nature.
Revival in 18th and 19th Century Germany 
During the Aufklarung (or German version of the eighteenth-century Enlightenment), the term "Humanität" was used to designate the intellectual, physical, and moral formation of "a better human being" (or Humanism) through immersion in the highest artistic and literary achievements of ancient Greece (harking back to the original connection of the term with the Ancient Greek concept of paideia). It was used, for example, by Johann Gottfried Herder in his Briefe zur Beförderung der Humanität and by Friedrich Schiller.
Humanitas as benevolence 
- The word occurs also in other Latin writers of the Classical period. For example, cultus atque humanitas (“culture and humanity"), meaning "civilization", appears in the opening sentences of Julius Caesar's Gallic Wars (1,1, 3), where Caesar calls the tribe of the Belgae the bravest, because farthest away from Romanized Southern France (Provence). It also occurs five times in the Rhetorica ad Herennium (for centuries erroneously attributed to Cicero). However, the concept was most fully elaborated by Cicero, who uses the word 299 times, accounting for about half of the 463 occurrences in all the other Classical Latin writers together. See Renato Oniga, "Humanitas" in Tulliana (2009) II. Cicero adapted the concept of humanitas from the Greek paidea (education), and gave it a distinctly Roman cast. Renato Oniga writes that according to a 1973 study by the German scholar Wolfgang Schadewaldt:
...l’essenza della humanitas romana sta propriamente nell’essere l’altra faccia di un insieme ordinato di valori molto precisi e severi, che facevano parte del codice di comportamento del cittadino romano fin dalle origini, e sono pressoché intraducibili in greco: la pietas (che è qualcosa di diverso dalla eusébeia), i mores (che non coincidono esattamente con l’ethos), e poi la dignitas, la gravitas, l’integritas, e così via. L’idea di humanitas riassumeva in sé tuttiquesti valori . . . ma nello stesso tempo li sfumava, li rendeva meno rigidi e più universali.
...the essence of Roman humanitas is the other side of an orderly complex of very distinct and severe values that had been part of the code of conduct of a Roman citizen from the outset, and are virtually untranslatable in Greek: pietas (which is different from eusébeia), mores (which do not coincide exactly with ethos), and dignitas, gravitas, integritas, and so on. The idea of humanitas subsumed all these values . . . simultaneously blurring their outlines, rendering them less rigid and more universal.
Early Italian humanism, which in many respects continued the grammatical and rhetorical traditions of the Middle Ages, not merely provided the old Trivium with a new and more ambitious name (Studia humanitatis), but also increased its actual scope, content, and significance in the curriculum of the schools and universities and in its own extensive literary production. The studia humanitatis excluded logic, but they added to the traditional grammar and rhetoric not only history, Greek, and moral philosophy, but also made poetry, once a sequel of grammar and rhetoric, the most important member of the whole group. —Paul Oskar Kristeller, Renaissance Thought II: Papers on Humanism and the Arts (New York: Harper Torchbooks, 1965), p. 178.
- Pro Archia paragraph 16.
- See Michael D. Reeve, "Classical Scholarship" in The Cambridge Companion to Renaissance Humanism, Jill Kraye, editor (Cambridge, 1996), p. 21.
- Peter Gay's citation of the phrase, Cum musis, etc., refers to an anecdote in the Tusculan Disputations, in which Cicero recounts how during a visit to Syracuse, in Sicily, he had chanced to discover the tomb of Archimedes, at that time unknown to the inhabitants of the city, but which he, Cicero, recognized from its description in a line of poetry he had memorized; and he contrasted the enduring fame of Archimedes, the mathematician, to the obloquy of the notorious Sicilian tyrant Dionysius the Elder, buried nearby: “Who is there who has had anything at all to do with the Muses, that is, with humanity and learning, who would not prefer to be this mathematician rather than that tyrant? If we look into their manner of life and employment, the mind of the one was nourished by seeking out and pondering theories, accompanied by the delight in his cleverness, which is the sweetest sustenance of souls, that of the other in murder and wrongdoing, accompanied by fear both day and night” (TD 5.64–5). This anecdote is one of the sources for the humanist commonplace that poetry is a more lasting monument than stone. See Mary Jaeger, ”Cicero and Archimedes Tomb", The Journal of Roman Studies, Vol. 92, (2002), pp. 51–52. The incident is recalled by Wordsworth:
Call Archimedes from his buried tomb
Upon the plain of vanished Syracuse,
And feelingly the Sage shall make report
How insecure, how baseless in itself,
Is the Philosophy, whose sway depends
On mere material instruments;—how weak
Those arts, and high inventions, if unpropped
By virtue.—He, sighing with pensive grief,
Amid his calm abstractions, would admit
That not the slender privilege is theirs
To save themselves from blank forgetfulness! —William Wordsworth (1770-1850), The Excursion (Book Eighth: "The Parsonage", lines 220-230)
- See Cicero, De Legibus, Book 1: 25.
- Peter Gay, The Enlightenment: The Rise of Modern Paganism (New York: W. W. Norton & Company,  1995), pp. 107–108.