Source : Webster's Revised Unabridged Dictionary (1913)

Invade \In*vade"\, v. t. [imp. & p. p. {Invaded}; p. pr. & vb.
   n. {Invading}.] [L. invadere, invasum; pref. in- in + vadere
   to go, akin to E. wade: cf. OF. invader, F. envahir. See
   {Wade}.]
   1. To go into or upon; to pass within the confines of; to
      enter; -- used of forcible or rude ingress. [Obs.]

            Which becomes a body, and doth then invade The state
            of life, out of the grisly shade.     --Spenser.

   2. To enter with hostile intentions; to enter with a view to
      conquest or plunder; to make an irruption into; to attack;
      as, the Romans invaded Great Britain.

            Such an enemy Is risen to invade us.  --Milton.

   3. To attack; to infringe; to encroach on; to violate; as,
      the king invaded the rights of the people.

   4. To grow or spread over; to affect injuriously and
      progressively; as, gangrene invades healthy tissue.

   Syn: To attack; assail; encroach upon. See {Attack}.

Invade \In*vade"\, v. i.
   To make an invasion. --Brougham.

Source : WordNet®

invade
     v 1: march aggressively into another's territory by military
          force for the purposes of conquest and occupation;
          "Hitler invaded Poland on September 1, 1939" [syn: {occupy}]
     2: to intrude upon, infringe, encroach on, violate; "This new
        colleague invades my territory"; "The neighbors intrude on
        your privacy" [syn: {intrude on}, {obtrude upon}, {encroach
        upon}]
     3: occupy in large numbers or live on a host; "the Kudzu plant
        infests much of the South and is spreading to the North"
        [syn: {overrun}, {infest}]
     4: penetrate or assault, in a harmful or injurious way; "The
        cancer had invaded her lungs"