Gains are specified in a number of ways depending on the particular amplifier,
usually small signal gain and gain have same meaning for say small signal transistor amplifier ,but for an power amplifier or other amplifier they would differ.
Please refer "Amplifier Fundamentals - By Simmons " for better explainations.
Small signal gain is the derivative of the output with respect to the input at a point on the input-output curve. In other words, it is the slope of the output-input curve at a point on the input-output curve.
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Gain, or "Large Signal Gain" is the output divided by the input at a point on the input-output curve. If the amplifier is perfectly linear, the two gains are equal.