An unobstructed path from a transmitting device to the receiver. Users are aware of line of sight when they aim their infrared (IR) remote control directly at the TV. Microwave transmissions between towers at the top of mountains and buildings require unobstructed line of sight. See
IR remote control.
Any Radio Frequency
Technically, line of sight refers to radio frequency (RF) transmission in general, including Wi-Fi and cellular. Although walls and other obstructions may attenuate (lessen) or block signals entirely, the receiving device must be in the radiation pattern emitted by the transmitter, which is also considered line of sight. See
RF.