1. The act of invading, especially the entrance of an armed force into a territory to conquer.
2. A large-scale onset of something injurious or harmful, such as a disease.
3. An intrusion or encroachment.

[Middle English invasioun, from Old French invasion, from Late Latin invāsiō, invāsiōn-, from invāsus, past participle of invādere, to invade. See invade.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.