What does "nun" mean?

Nun: (Zoöl.) (a) A white variety of domestic pigeons having a veil of feathers covering the head. (b) The smew. (c) The European blue titmouse. Gray nuns (R. C. Ch.), the members of a religious order established in Montreal in 1745, whence branches were introduced into the United States in 1853; -- so called from the color or their robe, and known in religion as Sisters of Charity of Montreal. -- Nun buoy. See under Buoy.