Glosa is a term that was often used by Spanish musicians in the 16th century to mean: a piece of music which was similar to a set of variations. Composers at that time often wrote collections of glosas. The term is related to the word “gloss”. They were comparing it to gloss (ornamentation) that a poet might put on his poem. They were often on religious themes.
Antonio de Cabezón was an important composer of glosas.
Glosa was also used by Spanish musicians to mean: musical ornamentation. This is the meaning in the title of a famous book of music for viola da gamba and harpsichord by Diego Ortiz called Trattado de glosas (1553).