Invasion is a word that comes from Latin. Originally, it meant that something goes in (as in military occupation) of a foreign territory. So far, the following uses are known:

• Military: Getting troops to take control of enemy territory. Often a state invades another to start a war, but invasions can happen anytime during war. An invasion differs from a raid, in which the attacker does not intend to remain.
• Others: Sometimes, biologists speak about an invasive species. There might be locust swarms invading, or that some algae spread far more rapidly in new territory.