1. An action, a gesture, or a sign used as a means of communication; such as, anything which serves to indicate, warn, direct, command; such as, a light, a gesture, an act, etc.: including a traffic signal or a signal to leave.
2. A piece of information communicated by an action, a gesture, or a sign.
3. Information transmitted by means of a modulated current or an electromagnetic wave and received by telephone, telegraph, radio, television, or radar.

1. Any sound that can be identified by listening to its acoustic characteristics; such as, pitch, beats, etc.
2. The audio portion of a television signal.

A signal designed for military use to produce a colored light or smoke, for the purpose of transmitting information.

The weakening of RF energy from an RFID tag or reader.

Water absorbs UHF energy, causing signal attenuation.