An analog input is a type of input signal that is continuous in nature, as opposed to being a discrete or digital signal. In other words, an analog input is a type of signal that can take on an infinite number of possible values within a given range. This is in contrast to a digital input, which can only take on one of a limited number of discrete values.

Analog inputs are commonly used in systems that require precise measurement of physical quantities, such as temperature, pressure, or sound. For example, a thermometer is an instrument that measures temperature using an analog input, typically in the form of a thermocouple or thermistor. The input signal is continuously monitored and the measured temperature is displayed on a dial or readout.

In order to be used by a digital system, an analog input must first be converted to a digital signal using an analog-to-digital converter (ADC). This process involves sampling the analog input at regular intervals and quantizing the samples to a finite number of discrete levels. The digital values can then be processed and analyzed by the system.