Insertion loss

1. The loss resulting from the insertion of a device in a transmission line, expressed as the reciprocal of the ratio of the signal power delivered to that part of the line following the device to the signal power delivered to that same part before insertion.

Note: Insertion loss is usually expressed in dB.

2. In an optical fiber system, the total optical power loss caused by insertion of an optical component, such as a connector, splice, or coupler.

Source: from Federal Standard 1037C