On reflection the definition might be a bit obscure, so here's a clue. The Council of Trent in 1545 was on the point of banning church music, as the words were inaudible; this composer demonstrated that this didn't have to be the case.
Thanks jws and bobsmum. Hope I'll be in earlier next week. (Not the most successful Christmas shopping - between the 4 of us who went together we bought 5 things, but only one of those is intended as a gift for someone else!)