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The root word is the primary lexical unit of a word, and of a word family (root is then called base word), which carries the most significant aspects of semantic content and cannot be reduced into smaller constituents.
Content words in nearly all languages contain, and may consist only of root morphemes. However, sometimes the term "root" is also used to describe the word minus its inflectional endings, but with its lexical endings in place. For example, chatters has the inflectional root or lemma chatter, but the lexical root chat. Inflectional roots are often called stems, and a root in the stricter sense may be thought of as a monomorphemic stem.
The traditional definition allows roots to be either free morphemes or bound morphemes. Root morphemes are essential for affixation and compounds. However, in polysynthetic languages with very high levels of inflectional morphology, the term "root" is generally synonymous with "free morpheme". Many such languages have a very restricted number of morphemes that can stand alone as a word: Yup'ik, for instance, has no more than two thousand.
The root of a word is a unit of meaning (morpheme) and, as such, it is an abstraction, though it can usually be represented in writing as a word would be. For example, it can be said that the root of the English verb form running is run, or the root of the Spanish superlative adjective amplísimo is ampl-, since those words are clearly derived from the root forms by simple suffixes that do not alter the roots in any way. In particular, English has very little inflection, and hence a tendency to have words that are identical to their roots. But more complicated inflection, as well as other processes, can obscure the root; for example, the root of mice is mouse (still a valid word), and the root of interrupt is, arguably, rupt, which is not a word in English and only appears in derivational forms (such as disrupt, corrupt, rupture, etc.). The root rupt is written as if it were a word, but it's not.
This distinction between the word as a unit of speech and the root as a unit of meaning is even more important in the case of languages where roots have many different forms when used in actual words, as is the case in Semitic languages. In these, roots are formed by consonants alone, and different words (belonging to different parts of speech) are derived from the same root by inserting vowels. For example, in Hebrew, the root gdl represents the idea of largeness, and from it we have gadol and gdola (masculine and feminine forms of the adjective "big"), gadal "he grew", higdil "he magnified" and magdelet "magnifier", along with many other words such as godel "size" and migdal "tower".
Secondary roots 
Consider the Arabic language:
- مركز [mrkz] or [markaza] meaning ‘centralized (masculine, singular)’, from [markaz] ‘centre’, from [rakaza] ‘plant into the earth, stick up (a lance)’ ( ركز | rkz).
- أرجح [rjh] or [ta'arjaħa] meaning ‘oscillated (masculine, singular)’, from ['urju:ħa] ‘swing (n)’, from [rajaħa] ‘weighed down, preponderated (masculine, singular)’ ( رجح | rjħ).
- محور [mhwr] or [tamaħwara] meaning ‘centred, focused (masculine, singular)’, from [mihwar] meaning ‘axis’, from [ħa:ra] ‘turned (masculine, singular)’ (حور | hwr).
- مسخر [msxr], تمسخر [tamasxara] meaning ‘mocked, made fun (masculine, singular)', from مسخرة [masxara] meaning ‘mockery’, from سخر [saxira] ‘mocked (masculine, singular)’ (derived from سخر[sxr])." Similar cases may be found in other Semitic languages such as Hebrew, Syriac, Aramaic, Maltese language and to a lesser extent Amharic.
Similar cases occur in Hebrew, e.om Israeli Hebrew מקמ √mqm ‘locate’, which derives from Biblical Hebrew מקום måqom ‘place’, whose root is קומ √qwm ‘stand’. A recent example introduced by the Academy of the Hebrew Language is מדרוג midrúg ‘rating’, from מדרג midrág, whose root is דרג √drg ‘grade’."
- iactito ‘to toss about’ derives from iacto ‘to boast of, keep bringing up, harass, disturb, throw, cast, fling away’, which in turn derives from iacio ‘to throw, cast’ (whose past participle is iactus).
- scriptito ‘to write often, compose’ is based on scribo ‘to write’ (<‘to draw lines, engrave with a sharp-pointed instrument’).
- dicto ‘to say often, repeat’ is from dico ‘to indicate, say, speak, tell’.
- clamito ‘to cry loudly/often, shout violently’ derives from clamo ‘call, shout’."
"Consider also Rabbinic Hebrew תרמ √trm ‘donate, contribute’ (Mishnah: T’rumoth 1:2: ‘separate priestly dues’), which derives from Biblical Hebrew תרומה t'rūmå ‘contribution’, whose root is רומ √rwm ‘raise’; cf. Rabbinic Hebrew תרע √tr` ‘sound the trumpet, blow the horn’, from Biblical Hebrew תרועה t'rū`å ‘shout, cry, loud sound, trumpet-call’, in turn from רוע √rw`." and it describes the suffix.
See also 
- Lemma (morphology)
- Morphological typology
- Morphology (linguistics)
- Phono-semantic matching
- Principal parts
- Proto-Indo-European root
- Radical (Chinese character) (this is more based upon a writing system than a spoken language)
- Semitic root
- Word family
- Word stem
- Zuckermann, Ghil'ad 2003, Language Contact and Lexical Enrichment in Israeli Hebrew, Houndmills: Palgrave Macmillan. ISBN 1-4039-1723-X. pp 65–66.
- Virtual Salt Root words and prefixes
- American Heritage Dictionary list of Indo-European roots
- Espindle - Greek and Latin Root Words