The Collaborative International Dictionary of English v.0.48:

Professor \Pro*fess"or\, n. [L., a teacher, a public teacher:
   cf. F. professeur. See Profess.]
   1. One who professed, or makes open declaration of, his
      sentiments or opinions; especially, one who makes a public
      avowal of his belief in the Scriptures and his faith in
      Christ, and thus unites himself to the visible church.
      "Professors of religion." --Bacon.
      [1913 Webster]

   2. One who professed, or publicly teaches, any science or
      branch of learning; especially, an officer in a
      university, college, or other seminary, whose business it
      is to read lectures, or instruct students, in a particular
      branch of learning; as a professor of theology, of botany,
      of mathematics, or of political economy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

professor
    n 1: someone who is a member of the faculty at a college or
         university [syn: professor, prof]