Command

1. An order for an action to take place.

2. In data transmission, an instruction sent by the primary station instructing a secondary station to perform some specific function.

3. In signaling systems, a control signal.

4. In computer programming, that part of a computer instruction word that specifies the operation to be performed. See command (computing) for detail.

5. Loosely, a mathematical or logic operator.

Source: from Federal Standard 1037C and from MIL-STD-188