The Collaborative International Dictionary of English v.0.48:

Lessor \Les"sor\ (l[e^]s"s[o^]r or l[e^]s*s[^o]r"), n. [See
   Lessee, Lease, v. t.] (Law)
   One who leases; the person who lets to farm, or gives a
   lease. --Blackstone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lessor
    n 1: someone who grants a lease [syn: lessor, lease giver]

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

LESSOR. contr. He who grants a lease. Civ. Code of L. art. 2647.