A Schmitt trigger changes its output state when its input voltage level rises above a certain reference voltage; however, the output does not switch back automatically when the input voltage level sinks again unless a second, lower reference voltage is crossed. This hysteresis makes Schmitt triggers useful for guarding against noise that would cause rapid switching back and forth between the two output states.
Schmitt triggers were first invented by US scientist O. H. Schmitt; today, they are typically built using operational amplifiers, and the reference voltage levels can be adjusted by controlling the resistances of R1 and R2:
The symbol for Schmitt triggers in electronic diagrams is a triangle with a hysteresis symbol: