The Collaborative International Dictionary of English v.0.48:

Migrant \Mi"grant\, n.
   1. A migratory bird, person, or other animal.

   2. A person who changes residence frequently in search of
      employment, especially farm labor, such as harvesting
      crops seasonally; also called migrant laborer or
      migrant worker. Sometimes the migrant worker is not a
      resident of the country in which the work is performed.
      [PJC]