Definition:
An insert card (sometimes simply called an insert) is a card that is not a part of the regular numbering system of a set of sports cards. Such cards either have their own numbering system or no number at all. Insert cards are usually found in packs and boxes of cards far less frequently than base cards.
Autographed cards and memorabilia cards are often insert cards when they are included in sports card products. By definition, a rookie card can't be an insert.
Examples: I had to open three packs before I found my first insert card.