A latch is a type of single-use switch, where the mechanism accepts a SET signal and remembers that that signal has come in. A RESET signal is later sent in on a different path; this separate input makes a latch different from a toggle circuit. If a SET signal has been detected since the last RESET, an output signal is generated in response. Often but not always, a different output signal is generated if there has been no previous SET signal.
The term is taken from electronic circuitry. Nominally a latch has two inputs, SET and RESET, and only one output. However, there are often two output signal paths, one the XOR inverse of the other.
In the context of stable glider and Herschel signals, a demultiplexer can be used as a latch. The Herschel input is the SET signal, and a glider is the RESET. If the latch has been set by a Herschel since the last RESET, the glider will be reflected 90 degrees; otherwise it passes through the demultiplexer unaffected.
- Flip-flop (electronics) at Wikipedia