A call option is a type of financial contract that gives the holder the right, but not the obligation, to buy an underlying asset at a predetermined price within a certain period of time. The crossword solution for the term 'call option' is TAILS, which is a reference to the two sides of a coin. This analogy is used to describe the two sides of a call option: the buyer and the seller. The buyer has the right to purchase the asset, while the seller has the obligation to sell the asset if the buyer exercises their option.