Q.Define signal to noise ratio.

The signal-to-noise ratio (SNR) is the single most direct measure of signal quality in a communication system. It compares how much wanted signal power is present with how much unwanted noise power accompanies it at any chosen point in a circuit:

The idea is intuitive. If the signal power is large and the noise power is small, the ratio is high and the message stands out clearly above the background — good reception. If the noise power grows or the signal weakens, the ratio falls and the message becomes harder to recover. In the ideal (but unreachable) case of zero noise, the SNR would tend to infinity. …