At the Money
An option is at-the-money if the strike price of the option equals the market price of the underlying security.
Investopedia Commentary
For example, if XYZ stock is trading at 75, then the XYZ 75 option is at-the-money. You can essentially think of this as the break-even point (when you don't take into account transaction costs).
Related Links
Options Basics Tutorial
Alternatives to Closing Below Intrinsic Value
See also: Deep in the Money, In the Money, Option, Out of the Money, Strike Price
Also spelled: At The Money, atthemoney, at-the-moneyATM