A cardholder is the person whose name is on a credit card — the one the bank made the card for. Banks, shops, and contracts use it in formal writing: “the cardholder must sign,” “cardholder agreement,” “notify us if you are not the cardholder.” In everyday talk people just say “I have a card” or “the person who owns the card.” The core sense is credit cards and money; people also stretch it to a debit card, an ID card, or a membership card, still meaning “the person who has that card,” not the plastic wallet that keeps cards in a bag.