The Yashts (Yašts) are a collection of twenty-one hymns in Younger Avestan. Each of these hymns invokes a specific Zoroastrian divinity or concept. Yasht chapter and verse pointers are traditionally abbreviated as Yt.
The word yasht derives from Avestan yešti, "for venerate" (see Christian Bartholomae`s Altiranisches wörterbuch, section 1298), and several hymns of the Yasna liturgy that "venerate by praise" are—in tradition—also nominally called yashts. These "hidden" Yashts are: the Barsom Yasht (Yasna 2), another Hom Yasht in Yasna 9-11, the Bhagan Yasht of Yasna 19-21, a hymn to Ashi in Yasna 52, another SaroshYasht in Yasna 57, the praise of the (hypostasis of) "prayer" in Yasna 58, and a hymn to the Ahurani in Yasna 68. Since these are a part of the primary litury, they do not count among the twenty-one hymns of the Yasht collection.
All the hymns of the Yasht collection "are written in what appears to be prose, but which, for a large part, may originally have been a (basically) eight-syllable verse, oscillating between four and thirteen syllables, and most often between seven and nine."
The Avesta has two hymns that were later titled Hom Yasht. The original is part of the Yasna liturgy and hence not counted as a Yasht. The other, Yasht 20, is a duplicate of the three verses of Yasna 9-11.