Each wallet service differs slightly, with some having several addresses for incoming transfers, and others which create new addresses for Bitcoin change. As such, a wallet typically handles multiple addresses.
It should be noted that the concept of a Bitcoin wallet is not included in the Bitcoin protocol. The simplest manifestation is ultimately the Bitcoin address; a wallet is used to conveniently manage addresses.
Some companies refer to their company's Bitcoin accounts as "wallets," so the definition is vague.