Sūtra (Sanskrit: सूत्र, Pāli: sutta, Ardhamagadhi: sūya) is an aphorism (or line, rule, formula) or a collection of such 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. 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.