The keyed expansion is minimum discernible signal — the weakest input a receiver can distinguish from its own noise, conventionally the level that produces an output equal to the noise alone, a 3 dB rise above the noise floor. That marks the threshold of detection, not a level at which the signal is necessarily usable for communication. The official pool uses both spellings, writing detectable in the E4C group heading and discernible in the keyed answer to this question, and the keyed wording is discernible.