Amex is the everyday short name for American Express, a big US company. It is best known for its charge cards and credit cards — plastic you use to pay, then settle the bill later — and it also does travel and other financial services. People say “I paid with Amex” or “Do you take Amex?” Shops sometimes put Amex on the door next to Visa and Mastercard. American Express is the full, more formal name; Amex is what you say in speech and in quick writing. Write it with a capital A. Though actually, AMEX in all capitals was also used for the American Stock Exchange, a different thing that happens to shorten the same way. In a café or a wallet, Amex means the card company.