Loss

In electronics, loss is the ratio of system output to system input. Calculation and units used are the same as with Gain, except that the output is less than the input. Loss is often expressed as a negative gain (if expressed in dB) or as a gain of less than one.

For example: if input=100mW and output=5mW then

loss = 5/100 = 0.05 = -1.3B (bel) or -13dB