Source : Webster's Revised Unabridged Dictionary (1913)

Locum tenens \Lo"cum te"nens\ [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.

Source : WordNet®

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