A prototype is a cognitive reference point, i.e the proto-image of all representatives of the meaning of a word or of a category. Thus, a robin or a sparrow can be regarded as a prototype or a "good example" of the category bird, whereas a penguin or an ostrich is a rather "bad example" of this category.
Accordingly, the members of a category can be graded according to their typicality. A "good" example only becomes such a one by virtue of its typical features. Defining a prototype as the bundle of typical features of a category, we can thus imagine birds as 'creatures that are covered with feathers, have two wings and two legs, and the majority of which can fly'. Therefore, a penguin is a less "good" bird, as it lacks some of the typical features, such the ability to fly. The features themselves can also be more or less typical, for example 'twittering' is less typical and specific to birds than 'flying'.
If an item shares at least some central features with the category prototype, we consider it as an example of this category. As a consequence, word meanings contain all the properties of cognitive categories: We can distinguish central and more peripheral meanings of a lexeme, and word meanings are not rigid, but there are often gradual transitions and fuzzy boundaries between them. Thus, prototype semantics is a 'more-or-less semantics', as opposed to the 'all-or-nothing' approach of structure-oriented feature semantics. However, this does not weaken the usefulness of a feature-based classification: The features belonging to a prototype of a category are the ones that are relevant for categorization.