In cold weather, we get water vapour coming out of the mouth as we breathe. What is that called?
This is simply called "breath", with no special word for the visible form. We often call this phenomenon "seeing one's breath":
I would call that steam (definition 2b: the mist formed by the condensation on cooling of water vapor ); even though it is most commonly associated with boiling, the word can be applied to any vapor created due to a heat differential.
(one example from the definition page: Their breath steamed the windows.)
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