| 1. an increase between two points of an electric, electromagnetic, or acoustic power|
2. the quantitative expression of a power increase by the ratio of the values at two points of a power or of a quantity related to power in a well defined manner
NOTE 1 – By extension, the word "gain" may represent the ratio of powers in a given situation and in a reference condition; for example the "gain of an antenna".
NOTE 2 – Gain is generally expressed in logarithmic units by a positive or negative value. When a gain has a negative value in logarithmic units, attenuation may be used instead of gain.