A survivor is a person who lives longer than another person, who lives past a certain point, or who lives through a certain situation.
In the context of the law, the term "survivor" is commonly used in wills and trusts to denote who assets should be passed to if one person outlives another, and if both people are beneficiaries of the trust or will.
The reason why the term "survivor" is used in wills and trusts, and sometimes in other legal contracts, is because sometimes if there are two or more people who are beneficiaries, or are otherwise involved in a contract, one or more of them can die. This could potentially create confusion. Often, however, a will may name two beneficiaries and designate assets to both of them, or to "the survivor." This means that if both of them are still alive, then they can both receive assets. However, if only one is alive, then only one will receive the assets.