The Collaborative International Dictionary of English v.0.48:

Instructor \In*struct"or\, n. [L., a preparer: cf. F.
   instructeur.] [Written also instructer.]
   One who instructs; one who imparts knowledge to another; a
   teacher.
   [1913 Webster]

WordNet (r) 3.0 (2006):

instructor
    n 1: a person whose occupation is teaching [syn: teacher,
         instructor]