The Collaborative International Dictionary of English v.0.48:

Attornment \At*torn"ment\, n. [OF. attornement, LL.
   attornamentum. See Attorn.] (Law)
   The act of a feudatory, vassal, or tenant, by which he
   consents, upon the alienation of an estate, to receive a new
   lord or superior, and transfers to him his homage and
   service; the agreement of a tenant to acknowledge the
   purchaser of the estate as his landlord. --Burrill.
   Blackstone.
   [1913 Webster]