Features are 'parasitic entities', that exist insofar their host exists. Typical examples of features are holes, bumps, boundaries, or spots of color. Features may be relevant parts of their host, like a bump or an edge, or dependent regions like a hole in a piece of cheese, the underneath of a table, the front of a house, or the shadow of a tree, which are not parts of their host. All features are essential wholes, but no common unity criterion may exist for all of them. However, typical features have a topological unity, as they are singular entities.