Definition:
A base card is a card that is included in and numbered as part of the regular run of cards in a sports card product. Base cards generally make up the majority of any given pack or box of sports cards.
This term is sometimes used interchangeably with the term common, but as some base cards are produced in lower quantities than others, this is not technically correct. All commons are base cards, but not all base cards are commons.
The complete group of all base cards in any product is known as the base set. Any card that is not part of a product's base set is known as an insert card.
Examples: Even when they aren't that valuable, it's always fun to open a pack and find a base card of your favorite player.