Polysemy

Definition
Polysemy is the phenomenon that a word acquires new usages which, over time, are likely to become more like new meanings.

Semantics
The phenomenon that a word has several different meanings which are closely related to each other. The ambiguity of church (either a building or an institution) is an instance of polysemy.

Example
The English word mouth refers first of all to the opening of the buccal cavity, but next to this it refers to the point where a river joins the sea.

Links
Utrecht Lexicon of Linguistics