The Collaborative International Dictionary of English v.0.48:

Manumission \Man`u*mis"sion\, n. [L. manumissio: cf. F.
   manumission. See Manumit.]
   The act of manumitting, or of liberating a slave from
   bondage. "Given to slaves at their manumission." --Arbuthnot.
   [1913 Webster]

WordNet (r) 3.0 (2006):

manumission
    n 1: the formal act of freeing from slavery; "he believed in the
         manumission of the slaves"

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

MANUMISSION, contracts. The agreement by which the owner or master of a
slave sets him free and at liberty; the written instrument which contains
this agreement is also called a manumission.
     2. In the civil law it was different from emancipation, which, properly
speaking, was applied to the liberation of children from paternal power.
Inst. liv. 1, t. 5 & 12; Co. Litt. 137, a; Dane's Ab. h.t.