Gain

In simple words, the gain of any circuit, system, or device is the simple ratio of output to the input of that system, circuit, or device where the output is always measured with respect to the input.

Actually, the term 'Gain' comes at the point when the output and input of a system are the same in nature. For example, in an audio amplifier circuit both input and output are the same, meaning both are audio signals and they have the same frequency, the same nature just difference is the output is more powerful than the input. The fact that how much the output is more powerful than the input, indicates the capacity of that amplifier which is represented by Gain.

Unlike voltage or current which is measured in volts and amperes respectively, the decibel, or simple dB for short, is just a ratio of two values, well actually the ratio of one value against another known or fixed value, so therefore the decibel is a dimensionless quantity, but does have the “Bel” as its units after the telephone inventor, Alexander Graham Bell.