A metaphorical extension is the "extension of meaning in a new direction" through popular adoption of an original metaphorical comparison. Metaphorical extension is almost a universal and natural process in any language undergone by every word. In general, it's not even perceived in everyday usage as meaning change. When it is least obvious, users don't even see it as extending the meaning of a word. Consider the example of illuminate: it originally meant "to light up" something dim or dark, but has evolved to mean "to clarify", "to edify". After a while these new meanings seem so natural as to be integral parts of the word, where senses such as "to celebrate" and "to adorn a page with designs" seem like more obvious additions.