(The Prakrit/Pali? form is "sutta".) It derives from the Sanskrit word for 'string'.
In Buddhism originally it was used for the written oral teaching of Buddha Sakyamuni, the founder of Buddhism. These teachings are mainly assembled in the second part of the Tripitaka which is called 'suttapitaka?'. Nowadays very often 'sutra' is taken in Buddhism in Japan and China for all kinds of Buddhist scriptures.
In Hinduism the 'sutra' are concise descriptions of the Brahmanas, the holy scriptures.