Attenuation refers to the loss of amplitude of a wave or a signal as it travels.
Two types of typical attenuators exist, optical attenuators and electrical attenuators. The former is used in fibre optic communications to test power level margins.
An electrical attenuator reduces the power of a signal without affecting the waveform.
Attenuation is the opposite of amplification. Whilst an amplifier raises the power of a signal, an attenuator reduces power or provides gain less than one.