The Collaborative International Dictionary of English v.0.48:

Alb \Alb\, n. [OE. albe, LL. alba, fr. L. albus white. Cf.
   Album and Aube.]
   A vestment of white linen, reaching to the feet, an
   enveloping the person; -- in the Roman Catholic church, worn
   by those in holy orders when officiating at mass. It was
   formerly worn, at least by clerics, in daily life.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alb
    n 1: a white linen liturgical vestment with sleeves; worn by
         priests