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 and is derived from the verbal root siv-, meaning to sew (these words, including Latin suere and English to sew, all ultimately deriving from PIE *siH-/syuH- to sew ), as does the medical term "suture."
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.