The Collaborative International Dictionary of English v.0.48:

Extravasate \Ex*trav"a*sate\, v. t. [imp. & p. p.
   Extravasated; p. pr. & vb. n. Extravasating.] [Pref.
   extra- + L. vas vessel: cf. F. extravaser. See Vase.]
   To force or let out of the proper vessels or arteries, as
   blood.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Extravasate \Ex*trav"a*sate\, v. i. [See Extravasate, v. t.]
   (Physiol.)
   To pass by infiltration or effusion from the normal channel,
   such as a blood vessel or a lymphatic, into the surrounding
   tissue; -- said of blood, lymph, etc.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

extravasate
    v 1: force out or cause to escape from a proper vessel or
         channel
    2: become active and spew forth lava and rocks; "Vesuvius erupts
       once in a while" [syn: erupt, belch, extravasate]
    3: geology: cause molten material, such as lava, to pour forth