A sutra is a literary rule or aphorism, or a collection of them. Sanskrit literature includes many collections of sutras.
The literary meaning of the word sutra is a bit different from the way Buddhists and Jains define it. In Buddhism, sutras are like scriptures, containing the teachings of the Buddha. Similarly, in Jainism, sutras are the sermons of a spiritual teacher. In Sanskrit, sutra means "thread," and traditional ancient literary sutras attempted to weave knowledge, threadlike, around and into their few simple words or syllables.