1. An option contract that gives the holder the right to buy a certain quantity (usually 100 shares) of an underlying security from the writer of the option, at a specified price (the strike price) up to a specified date (the expiration date). also called call option. 2. The act of exercising a call option. 3. The right to redeem a callable bond before its scheduled maturity. 4. In banking, a demand to repay a security loan immediately. 5. Mortgage: To announce that the debt in its entirety is due at once.