Signal-to-noise ratio (SNR)
Signal-to-noise ratio is the single number that says how clearly a signal stands out above the ever-present hiss. It's the ratio of signal power to noise power, almost always quoted in decibels: 0 dB means signal and noise are equally strong (barely usable), 20 dB means the signal is 100× stronger than the noise (comfortable), 40 dB is excellent. Picture trying to hear a friend across a noisy café — SNR is exactly how much louder their voice is than the surrounding chatter.
SNR is the master dial of a communication link. Together with bandwidth it sets the Shannon capacity — the absolute maximum error-free data rate. A high SNR lets you use a dense constellation like 1024-QAM for blistering speed; a low SNR forces you down to rugged BPSK and heavy error-correcting codes just to survive. Every engineering choice — antenna gain, transmit power, distance, modulation order — ultimately shows up as a change in the SNR the receiver sees.
In digital systems engineers often prefer Eb/N0 — energy per bit over noise spectral density — because, unlike raw SNR, it lets you fairly compare modulation schemes that send different numbers of bits per symbol.