Alienation is a term developed by the young Marx. It essentially means separation from. In order to be alienated, therefore, a thing first has to be part of the thing from which it is alienated. For example, Man can be alienated from Nature, because Man arises as part of Nature. Labour can be alienated from the worker, because the labour was a part of the worker, and so on.
I discussed these issues in my blog Alienation Of Labour.
A detailed list of Marx and other Marxists writings on Alienation is provided – here.