Definition of “steam” - Learner’s Dictionary

steam

noun [ U ] us uk /stiːm/
Make a slit in the pastry to allow the steam to escape.The carriages were pulled by a steam locomotive.The bathroom was full of steam.The valve allows the steam to escape.The steam produced in the tubes collects at the top of the drum.

B2 the gas that water produces when you heat it

(Definition of “steam noun” from the Cambridge Learner's Dictionary © Cambridge University Press)