The Collaborative International Dictionary of English v.0.48:

Locum tenens \Lo"cum te"nens\n. [L., holding the place; locus
   place + tenens, p. pr. of tenere to hold. Cf. Lieutenant.]
   A substitute or deputy; one filling an office for a time.
   [1913 Webster]

WordNet (r) 3.0 (2006):

locum tenens
    n 1: someone (physician or clergyman) who substitutes
         temporarily for another member of the same profession [syn:
         locum tenens, locum]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

LOCUM TENENS. He who holds the place of another, a deputy; as A B, locum
tenens of C D, mayor of the city of Philadelphia.