means A hostile entry of armed forces into a territory to conquer or occupy it — or, more loosely, any overwhelming intrusion or arrival in great numbers.

from From Latin 'invadere,' to go into or attack — built from 'in-' (into) plus 'vadere' (to go, to walk), the same restless 'vadere' that strides through 'evade' (go out of) and 'pervade' (go through everything). So an invasion is, at root, simply a marching-in — the verb of taking a step, weaponized.