A sutra (SanskritsūtraPali: sutta, Ardha Magadhi: sūya) is an aphorism or a collection of aphorisms in the form of a manual or, more broadly, a text in Hinduism or Buddhism. Literally it means a thread or line that holds things together and is derived from the verbal root siv-, meaning to sew. The word "sutra" was very likely meant to apply quite literally to these texts, as they were written down in books of palm leaves sewn together with thread. This distinguishes them from the older sacred Vedas, which until recently were only memorised, never committed to paper.
In ancient Indian literature, sutra denotes a distinct type of literary composition, based on short aphoristic statements, generally using various technical terms. This literary form was designed for concision, as the texts were intended to be memorized by students in some of the formal methods of scriptural and scientific study (Sanskrit: svādhyāya). Since each line is highly condensed, another literary form arose in which commentaries (Sanskrit: bhāṣya) on the sutras were added, to clarify and explain them. For discussion of the literary form for sutras, their terse nature as a summary of ideas for memorization, and the rise of the commentorial literary form as an adjunct to sutras, see: Tubb & Boose 2007, pp. 1–2.
In Brahmin lineage, each family is supposed to have one gotra and one Sutra, meaning that a certain Veda (Śruti) is treasured by this family in way of learning by heart. One of the most famous definitions of a sutra in Indian literature is itself a sutra and comes from the Vayu Purana:
alpākṣaraṃ asandigdhaṃ sāravad viśvatomukham
astobhaṃ anavadyaṃ ca sūtram sūtravido viduḥ.
Of minimal syllabary, unambiguous, pithy, comprehensive,
continuous, and without flaw: who knows the sutra knows it to be thus.
In Jainism, sutra or suya refers to canonical sermons of Mahavira contained in the Jain Agamas and to some later (post-canonical) normative texts.
Some scholars consider that the Buddhist use of sutra is a faulty Sanskritization of the Prakrit or Pali word sutta and that the latter actually represented Sanskrit sūkta, "well spoken, good news". The early Buddhist sutras do not present the aphoristic, nearly cryptic nature of the Hindu sutras even though they also have been designed for mnemonic purposes in an oral tradition. On the contrary, they are most often lengthy, with many repetitions which serve the mnemonic purpose of the audience. They share the character of sermons of "good news" with the Jaina sutras, whose original name of sūya in Ardha Magadhi can derive from Sanskrit sūkta, but hardly from sutra.