Symbolism, Imagery, Allegory
It's worth pointing out that The Zoo Story as a title is important because it's not what the play is about. Jerry keeps saying he'll talk about the zoo, but he never does. The play is about what does not happen at the zoo. The title is a head-fake if we've ever seen one, and we talk about it more in our "What's Up With the Title?" section—shocking, right?
Albee is the kind of writer who uses even a head-fake as a symbol, though. In this case, the fact that you never see the zoo is a tip-off that the play isn't about what it says it's about; it's allegorical or symbolic instead. This is not about two guys who have a fight over a bench. It's bigger and more mysterious than that. The zoo symbolizes the fact that the zoo is not the zoo. That's how the Theater of the Absurd rolls. (See "Genre.")