Hydration refers to filling an existing object with data. It is not a synonym for serialization.
With respect to the more generic term hydrate Hydrating an object is taking an object that exists in memory, that doesn't yet contain any domain data ("real" data), and then populating it with domain data (such as from a database, from the network, or from a file system).
From Erick Robertson's comments on this answer:
deserialization == instantiation + hydration