Wiktionary
n. An indicator used to measure the strength of the radio signal, often provided on communications receivers.
Wikipedia
An S meter (signal strength meter) is an indicator often provided on communications receivers, such as amateur radio receivers or shortwave broadcast receivers. The scale markings are derived from a system of reporting signal strength from S1 to S9 as part of the RST code. The term S unit can be used to refer to the amount of signal strength required to move an S meter indication from one marking to the next.
Usage examples of "s meter".
Nenda went to sit on the Zardalu's meter-thick mid-section, and Kallik at once hurried over to crouch at his feet.
It grew louder and more rhythmic as Sayeesa'a screams weakened, and Matt realized her body had begun to twist in time to the old man's meter.