In the broadest definition, a sensor is an object whose purpose is to detect events or changes in its environment, and then provide a corresponding output. [1]

If we follow the above definition given on the Wikipedia then some important and interesting issues arise:

Is sensor not a device that behaves in certain manner to some energy stimulus and thus allows us to connect outer energy to the autonomous system (robot) being developed?

Should a sensor always transduce to electronic energy form?

e.g. a gear lever in a bike is not a sensor? it senses (may be pressure) and thus allows the system (the bike) to sense some outside stimulus.

Please feel free to define sensor in a different manner if you disagree with [1].

[1] https://en.wikipedia.org/wiki/Sensor