Noun
float switch (plural float switches)
A sensor that detects the level of liquid in a tank and can react to changes in the measurement.
In the example, the physical state of the float switch contacts must be considered when choosing "normally open" or "normally closed" symbols in the ladder diagram. Source: Internet
When the water level is below both switches, the float switch physical contacts are both closed, and a true (logic 1) value is passed to the Fill Valve output. Source: Internet