Nationalise is the British spelling of nationalize. The meaning is the same: a government takes a big industry — water, gas, the railways, oil, sometimes banks — out of private hands and makes the state the owner or the one in charge. It may buy the companies, or it may simply take control. This is a politics-and-news word, not kitchen-table talk. People who want a smaller state often spit the word out as a warning; people who want public services in public hands use it as a plan. British English often writes -ise on this family of verbs; American English usually writes -ize. You have not changed the idea, only the letters. The matching noun is nationalisation (British) or nationalization (American). The opposite move is to privatise / privatize.

國有化（動詞；英式拼法，美式多寫 nationalize）