A) A measure of how well a load is matched to a transmission line
Standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line.
SWR is used as a measure of efficiency for radio transmission lines. SWR is important since it provides an indication of the radio waves reflected back toward the transmitter. Under ideal conditions, a radio transmission cable would have an SWR of 1:1 (1 to 1) indicating that all power is being delivered to the antenna and none being reflected back to the transmitter. An infinite SWR indicates all power is being reflected back to the transmitter (with none being delivered to the antenna). High SWR ratios are to be avoided and can indicate RF is being radiated in the shack, RF is being wasted as heat, and equipment failure may result.