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The term "discrete signals" in the context of PLC (Programmable Logic Controller) inputs refers to signals that are either ON or OFF. This means that discrete signals can only exist in two distinct states, typically represented as binary values: one state (often represented as 1 or "ON") indicates that a machine component is activated or engaged, while the other state (represented as 0 or "OFF") indicates that it is deactivated or disengaged.

This characteristic makes discrete signals particularly useful in applications where binary decisions or control logic is necessary, such as in controlling motors, sensors, or switches. Discrete inputs are commonly found in automation systems, allowing PLCs to process clear and straightforward information regarding the operating status of various devices in a system.

The other options do not align with the definition of discrete signals. Signals that vary continuously relate to analog signals, which can represent an infinite number of values within a specific range. Signals that represent more than one value would imply a level of complexity beyond just ON or OFF states. Lastly, signals that are not measurable would be outside the scope of what is considered in digital control systems, as discrete signals must have definable, measurable states.