Definition:
A parallel is a sports card that is identical to a base card or insert card in the same product with the exception of one or more (sometimes minor) visual elements. Such cards usually have smaller print runs than the original and occasionally have autographs or memorabilia swatches that the original did not have. These differences can lead to the parallel being worth more than the original card.
A single card can have multiple levels of parallel cards, each with different visual tweaks and print runs. A set that includes parallels for only some of the cards in the original set is called a partial parallel.
Examples: I sometimes get confused when one card has blue, green, red, gold and platinum parallels.