'Rookie Card' - what is true definition?

What is the true definition of a 'Rookie Card'?

You see the term all too often these days.

As many of you know, I have a New York Jets jersey signed by Mark Sanchez. I'm hoping to pick up a selection of cards to put in a frame with it - ideally a true rookie card since it's his first NFL year.

You search "Mark Sanchez rookie auto" on eBay and get four pages of results all with different cards, companies and prices.

Any ideas or discussion on this matter?

What they mean is rookie 'year' auto. Only the first common from any set is considered a true RC. These days that can mean a common or a common auto jersey such as Bron's exquisite.