The word “discrete” means individual or distinct. In engineering, a “discrete” variable or measurement refers to a true-or-false condition. Thus, a discrete sensor is one that is only able to indicate whether the measured variable is above or below a specified setpoint.
Discrete sensors typically take the form of switches, built to “trip” when the measured quantity either exceeds or falls below a specified value. These devices are less sophisticated than so-called continuous sensors capable of reporting an analog value, but they are quite useful in industry.
Many different types of discrete sensors exist, detecting variables such as position, fluid pressure, material level, temperature, and fluid flow rate. The output of a discrete sensor is typically electrical in nature, whether it be an active voltage signal or just resistive continuity between two terminals on the device.