Marty...

The term "mastering" has gotten abused severely in the past 20 years (with the advent of home recording). The video that Noel posted (by Graham) explains it perfectly. Mastering is a term that should apply to a collection of songs - like for a CD release. It is the Mastering engineer's job to make them sound like a cohesive unit. The only reason to apply the term "mastering" to a single song is to make it "fit" into whatever sonic field it might play in - as he states - get it to the level of the CDs or radio songs it will compete against...so it doesn't stand out as "out of place" when listened to in context.

Everything else is mixing.