Verb: When describing the act of going into a state of inactivity during winter, like animals do.
* Example: "The bears hibernate in caves during the winter months."
Noun: When referring to the state of inactivity itself.
* Example: "The animal's hibernation period lasts for several months."
Other possibilities:
* Adjective: While less common, "hibernate" can sometimes be used as an adjective to describe something related to hibernation.
* Example: "The bear's hibernate den is located in a remote cave."
So, the specific part of speech depends on the context in which "hibernate" is used.