Apartment 5A is Alicia's apartment, directly above Sheldon's and Leonard's apartment (4A). She moved in during "The Dead Hooker Juxtaposition", and the apartment has not appeared in any other episodes since.
Sheldon was terrified at the prospect of new upstairs tenants whom he had never met, saying he preferred the old ones because they were always quiet. Alicia was making quite a bit of noise upstairs while supposedly having sex with a producer from CSI. They don't mention Apartment 5A in the previous episodes, or after the episode it appeared in.
The apartment is almost identical in shape and configuration to the apartment that Sheldon and Leonard live in.