To change a whole planet — its air, its temperature, its water — until human beings could live there without a spacesuit. The idea is to make somewhere else more like Earth. People talk about terraforming Mars: thickening the atmosphere, warming it up, melting the ice so there is liquid water.

It is a verb: 'to terraform Mars', 'a terraformed moon', 'the terraforming of Venus'. The -ing form, 'terraforming', works as a noun and is the form you will see most.

This word was born in science fiction — an American writer coined it in 1942 — and it still lives mostly there: novels, films, and strategy games where you slowly turn a red planet green. But scientists now use it seriously too, usually to argue about whether it is possible at all, and how many centuries it would take.

People also borrow it as a joke or an exaggeration for making a place fit you: 'they've terraformed the whole neighbourhood into a shopping district'. That is a stretched use, and it always sounds a little playful.

One warning for anyone in tech: Terraform, with a capital T, is also the name of a well-known software tool for setting up servers. Completely unrelated to planets, and if you search the word online, that is mostly what you will find.