• (a.) Responsory; antiphonal.
Re`spon·so'ri·al adjective Responsory; antiphonal. J. H. Newman. 
a responsorial chant emphasizes the `response ` between a soloist and the choir. Responsorial chants tend to be extremely elaborate, particularly in their second section (known as the verse) where the soloist performs the bulk of the musical material and the choir enters only at the end.
in plainsong, a section for solo voice followed by a section for chorus in unison.