Acclimate, acclimatise, and acclimatize share one of their definitions: to accustom or become accustomed to a new environment or situation. In British and Australian English, acclimatise appears most often and is generally preferred over the alternatives. In American English, acclimatize is recommended by many dictionaries and usage guides, yet acclimate, which is actually the older word, is far more common. Canadian writers tend to use acclimatize.
The words’ corresponding nouns are acclimation, acclimatisation, and acclimatization.
These British and Australian publications prefer acclimatise:
Like most fighters on this card, Pearson arrived two weeks before the event to acclimatise to being on the other side of the globe. [Telegraph]
I’d spent the previous day resting, after arriving from Delhi, in an effort to acclimatise to the altitude of 3500 metres … [Sydney Morning Herald]
They entered Tibet through north-west India, acclimatising slowly. [Guardian]
American publications prefer acclimate by a wide margin—for example:
It’s best to thin these trees slowly over a period of a few years, allowing the tree to acclimate slowly. [Chicago Tribune]
Tomlinson worked with the existing engineers to help acclimate them to coding in Java rather than the Apple-preferred language, Objective-C. [Wired]
You acclimate quickly, and pretty soon you don’t want to leave. [Wall Street Journal]