locum tenens

from 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}]

from 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]

from 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.

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