Is UTF-8 an encoding or a character set?

I thought that the name of the character set was "Unicode" and that "UTF-8" was the name of a particular encoding of the Unicode character set, but I often see the terms "encoding" and "charset" used interchangeably when referring to UTF-8.

For example,

<meta charset="UTF-8">

vs

<?xml version="1.0" encoding="UTF-8" ?>