There are several synonyms for the phrase "in the adjacent apartment". These include "in the neighboring apartment", "in the next-door apartment", "in the adjoining apartment", "in the nearby apartment", and "in the opposite apartment". Each of these phrases can be used interchangeably to describe someone living in an apartment adjacent to one's own. It is important to note that context can influence which synonym is most appropriate to use. For example, "in the opposite apartment" may be more apt if the apartments are located across from one another rather than beside. The synonym chosen should accurately convey the intended meaning.