It's a perfectly cromulent word, but at the link only "adaptation" appears.

> It’s a perfectly cromulent word

TIL: https://www.merriam-webster.com/words-at-play/what-does-crom...

I was sure it wasn't a real word but you're correct, it's an alternative (but less common) spelling.

"less common" is a bit of an understatement[0]. I'd put it in the "common misspelling" category. Though both versions seem to have been around since before English really had consistent spelling.

[0] https://books.google.com/ngrams/graph?content=adaption%2Cada...

The page at the other end of the hyperlink uses the word "adaptation" twice and "adaption" never.