The Collaborative International Dictionary of English v.0.48:

Disseizin \Dis*sei"zin\, n. [OF. dessaisine.] (Law)
   The act of disseizing; an unlawful dispossessing and ouster
   of a person actually seized of the freehold. [Written also
   disseisin.] --Blackstone.
   [1913 Webster]

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

DISSEISIN, torts. The privation of seisin. It takes the seisin or estate
from one man and places it in another. It is an ouster of the rightful owner
from the seisin or estate in the land, and the commencement of a new estate
in the wrong doer. It may be by abatement, intrusion, discontinuance, or
deforcement, as well as by disseisin, properly so called. Every
dispossession is not a disseisin. A disseisin, properly so called, requires
an ouster of the freehold. A disseisin at election is not a disseisin in
fact; 2 Prest. Abs. tit. 279, et seq.; but by admission only of the injured
party, for the purpose of trying his right in a real action. Co. Litt. 277;
3 Greenl. 316; 4 N. H. Rep. 371; 5 Cowen, 371; 6 John. 197; 2 Fairf. 309, 2
Greenl. 242; 5 Pet. 402; 6 Pick. 172.
     2. Disseisin may be effected either in corporeal inheritances, or
incorporeal. Disseisin of things corporeal, as of houses, lands, &c., must
be by entry and actual dispossession of the freehold; as if a man enters, by
force or fraud, into the house of another, and turns, or at least, keeps him
or his servants out of possession. Disseisin of incorporeal hereditaments
cannot be an actual dispossession, for the subject itself is neither capable
of actual bodily possession nor dispossession. 3 B1. Com. 169, 170. See 15
Mass. 495 6 John. R. 197; 2 Watts, 23; 6 Pick. 172 1 Verm. 155; 11 Pet. R.
41; 10 Pet. R. 414; 14 Pick. 374; 1 Dana's R. 279; 2 Fairf. 408; 11 Pick.
193; 8 Pick. 172; 8 Vin. Ab. 79; 1 Swift's Dig. 504; 1 Cruise, *65; Arch.
Civ. Pl. 12; Bac. Ab. h.t.; 2 Supp. to Ves. Jr. 343; Dane's Ab. Index, h.t.;
1 Chit. Pr. 374, note (r.)