At-the-money is a term from options trading.
The option is said to be at-the-money when the underlying asset is trading at the same price as the option.
For example, if a stock is trading at $50 and the option strike price is $50, the option is at-the-money.
An at-the-money option has no intrinsic value, only time value. The option has time value until it reaches its expiry date.
As a rough approximation, the price of an at-the-money option increases with the square root of its time to maturity.
John C. Hull in The Harriman Book of Investing Rules, edited by Philip Jenks and Stephen Eckett.
Options can also be in-the-money or out-of-the-money.