True bypass
An effects pedal that, when switched off, wires its input straight to its output with a mechanical switch, so the circuit drops out entirely.
The alternative is buffered bypass, where the signal always passes through a low-output-impedance stage even with the effect off — Boss does this across its whole compact range. True bypass uses a triple-pole switch, the 3PDT, to route the signal clean around the circuitry.
The argument is really about cable capacitance. A guitar pickup has high impedance, and together with the capacitance of the cables it forms a low-pass filter. Chain ten true-bypass pedals and add seven metres of lead to the amp, and the total wire is long enough to dull the treble — what guitarists call tone suck. The sensible fix is one good buffer at the start of the chain and true bypass for the rest. The commercial fix is printing "true bypass" on every box.