1. The act of invading, especially the entrance of an armed force into a territory to conquer.
2. The entry into bodily tissue and subsequent proliferation of an injurious entity, such as a pathogen or tumor.
3. An intrusion or encroachment: Your reading her diary was an invasion of her privacy.
[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, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.