Definition
A comparison between the strength of a desired signal and the level of background noise present with it, expressed as a ratio. A higher ratio means the signal is clearly stronger than the noise; a lower ratio means the noise is interfering with or masking the signal.
Plain English
How much louder or stronger the wanted signal is compared to the unwanted background interference. The bigger the gap, the cleaner and more reliable the signal.
Context Anchor
Seen in discussions of radios, navigation receivers, radar, GPS, and other aircraft electronic systems.
Derivation
Built from three plain words: signal (the information you want), noise (random interference you do not want), and ratio (a comparison between two amounts). The phrase simply describes how the wanted information compares against the unwanted clutter.
Why Pilots Care
A low ratio makes voice transmissions or navigation signals hard to read, increasing the risk of missed instructions or unreliable course guidance.
Analogy
Think of trying to hear someone speak in a quiet room versus a noisy bar. Same voice, same words -- but in the bar, the background noise is so close to the level of their voice that you miss half of what they say. That gap between the voice and the background is the signal-to-noise ratio.
Intuition Check
Signal-to-noise ratio is not just the strength of the signal. It is the strength of the useful signal compared with the unwanted background interference.
Example Sentence 1
The VOR indication became unreliable at low altitude because the signal-to-noise ratio dropped as the aircraft moved farther from the station.
Example Sentence 2
Heavy precipitation reduced the signal-to-noise ratio on the ADF, making the NDB station unreadable.