What is "ternary form"

n. (context music English) A style of composition in which one contrasting section of music is written between two similar outer sections.

Ternary form, sometimes called song form, is a three-part musical form where the first section (A) is repeated after the second section (B) ends. It is usually schematized as A–B–A. Examples include the da capo aria "The trumpet shall sound" from Handel's Messiah, Chopin's Prelude in D-Flat Major (Op. 28) and the opening chorus of Bach's St John Passion.