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]

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

ATTORNMENT, estates. Was the agreement of the tenant to the grant of the
seignory, or of a rent, or the agreement of the donee in tail, or tenant for
life, or years, to a grant of a reversion or of a remainder made to another.
Co. Litt. 309; Touchs. 253. Attornments are rendered unnecessary, even in
England, by virtue of sundry statutes, and they are abolished in the United
States. 4 Kent, Com. 479; 1 Hill. Ab. 128, 9. Vide 3 Vin. Ab. 317; 1 Vern.
330, n.; Saund. 234, n. 4; Roll. Ab. h.t.; Nelson's Ab. h.t.; Com. Dig. h.t.