In programming, a value that you pass to a routine. For example, if SQRT is a routine that returns the square root of a value, then SQRT(25) would return the value 5. The value 25 is the argument.
Argument is often used synonymously with parameter, although parameter can also mean any value that can be changed. In addition, some programming languages make a distinction between arguments, which are passed in only one direction, and parameters, which can be passed back and forth, but this distinction is by no means universal.
An argument can also be an option to a command, in which case it is often called a command-line argument.