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The term water vapor planet is likely to be used for an extra-solar planet in which water vapor has been detected in its atmosphere (i.e., like Earth in this respect). Another term is steam planet, which more likely would be used if the temperature appears to be in the range of steam at an atm, i.e., steam as we are used to it, or if the atmosphere is primarily water vapor (e.g., a steam atmosphere).
The terms may also be used for a theoretical planet composed basically of water vapor (perhaps with a small core); these are not necessarily presumed to exist, but even so, serve as one of a number of theoretical single-constituent planets often compared with observed exoplanets to provide clues to their mix of constituents.