from Red Pine, The Heart Sutra, pp. 38-40:

“The Sanskrit word sūtra is usually interpreted as deriving from the root siv, meaning “to sew,” and as referring to a “thread” that holds things together, like the English word suture. However, some scholars have suggested that it might instead come from sukta, meaning “wise saying.” Whatever its derivation, sutra was used by Brahmans and Jains as well as Buddhists to denote a scripture.

“According to the traditional account, Buddhist sutras date back to the First Council, which was held in Rajgir in the months immediately after the Buddha’s Nirvana in 383 B. C. Many scholars now believe such an account was a later fabrication by early Buddhist sects anxious to authenticate their selections and interpretations of the Buddha’s teachings. But whether or not such an event took place, these early sects applied the word sūtranot only to discourses of the Buddha but also to discourses on the Abhidharma by later followers as well. As time went on, however, the word shastra, meaning “investigation,” was used for Abhidharma texts, and the word sutra was reserved for sermons of the Buddha or disciples empowered by him to speak on his behalf.

“In the case of the Heart Sutra, the text before us was not considered a ching or “sutra” until Hsuan-tsang’s translation of 649. Prior to that, the text was considered a mantra or dharani, as reflected in the earlier translations of the title by Chih-ch’ien and Kumarajiva. Also, it is worth noting that none of our extant Sanskrit copies includes the word sutra in the title, and it is only reflected in the Chinese and Tibetan. However, since it has been customary for the past thousand years or so to refer to this as a “sutra,” I have retained this word in the title.”