To enter somewhere forcefully and usually without permission, whether literally (an army invading a country) or figuratively (a thought invading your mind).
From Latin 'invadere' (to go into), combining 'in-' (into) + 'vadere' (to go). The word maintains the same meaning from ancient Rome to modern times.
Invade is one of those words that works perfectly in both military and everyday contexts: an army invades a country, a virus invades cells, a thought invades your mind, a smell invades a room. This metaphorical flexibility is what makes it so useful—it captures the essence of 'something unwanted entering a space' whether that space is geography, biology, or psychology.
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