The Collaborative International Dictionary of English v.0.48:

Alumnus \A*lum"nus\, n.; pl. Alumni. [L., fr. alere to
   nourish.]
   A pupil; especially, a graduate of a college or other
   seminary of learning.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alumnus
    n 1: a person who has received a degree from a school (high
         school or college or university) [syn: alumnus, alumna,
         alum, graduate, grad]