What does irredentism mean?

I keep looking up definitions and they are all very complicated. Could you give me a simple meaning please. Thanks

From the Wikipedia

"irredentism is any position advocating annexation of territories administered by another state on the grounds of common ethnicity and/or prior historical possession, actual or alleged."

When the US "annexed" the Texas territory based on the fact that it was being settled by Americans, that would be irredentism