The ratio is often expressed in decibels (dB).
For example, if output=500mW and input=5mW then
gain=500/5 = 100 = 2B (bel) or 20dB
In finance, gain is profit or increase in value of an investment such as stock and bond. Gain is calculated by fair market value or proceed from sale of the investment minus the sum of purchase price and all costs associated with it. If the investment is not converted into cash or another asset the gain is called unrealized gain.