The Collaborative International Dictionary of English v.0.48:

fellatio \fel*la"ti*o\ (f[e^]l*l[=a]"sh[=e]*[-o];
   f[e^]l*l[=a]"t[=e]*[-o]), n.
   a form of sexual activity between two persons in which the
   penis of one is stimulated by the other's mouth, especially
   until orgasm is achieved. It is a form of oral sex. Compare
   cunnilingus. The act is also called by the slang term blow
   job.
   [PJC]

WordNet (r) 3.0 (2006):

fellatio
    n 1: oral stimulation of the penis [syn: fellatio,
         fellation]