Mapping is used for key-to-value mapping. Mappings can be seen as hash tables that are virtually initialized such that every possible key exists and is mapped to a default value. The default value is all zeros. The key is never stored in a mapping, only the keccak256 hash is used for value lookup. Mapping is defined just like any other variable type. Take a look at this code:

contract Gotham {

    struct Batman {
        string friends;
        string foes;
        int funds;
        string fox;
    }

    mapping (address => Batman) Catwoman;
   address[] public Batman_address;
}

The preceding code example shows that Catwoman is being initialized as a mapping.