Definition of "cassock"

It was a joy to snatch some brief respite, and find himself in the rectory drawing–room. Listening here was as pleasant as talking; just to watch was pleasant. The young priests who lived here wore cassocks and birettas; their faces were fine and mild, yet really strong, like the rector's face; and in their intercourse with him and his wife they seemed to be brothers.

1918, W[illiam] B[abington] Maxwell, chapter X, in The Mirror and the Lamp, Indianapolis, Ind.: The Bobbs-Merrill Company

In one of the aisles there was an elaborately carved confessional box and I recognised the village priest in his heavy mountain boots and black cassock as he entered it and drew the dark velvet curtains behind him.

1956, Delano Ames, chapter 13, in Crime out of Mind‎

When leading worship, Elsie usually wore a long black cassock and preaching bands. This would, of course, save her from the worry of choosing a suitable costume.

2014, Alan Argent, Elsie Chamberlain: The Independent Life of a Woman Minister, Routledge, page 203