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.

This short article about the military can be made longer. You can help Wikipedia by adding to it.