The Collaborative International Dictionary of English v.0.48:

Tumescence \Tu*mes"cence\, n. [L. tumescens, -entis, p.pr. of
   tumescere to swell up, v. incho. fr. tumere to swell.]
   The act of becoming tumid; the state of being swollen;
   intumescence; -- applied especially to the state of swelling
   of the vascular tissue in the male and female sex organs when
   they have been stimulated to readiness for sexual
   intercourse.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

tumescence
    n 1: tumidity resulting from the presence of blood or other
         fluid in the tissues