Apportionment means the distribution or allocation of shares, and can refer to a share of a benefit or liability.

When used in relation to property law, it usually refers to that fact that when a lessee (someone that rents a property) is evicted from (or gives up possession of part of) the property leased to him, he's liable to pay only a part of the rent. In other words the rent is 'apportioned' accordingly.