What does the word encoder mean?！

The encoder is an electromechanical device that can measure displacement.
Encoders are normally digital displacement transducers, consisting of a mechanical element and a sensing head, typically of optical type. The mechanical element can be a disc (for rotary type encoders) or a ruler (for linear type encoders) with deposited or carved patterns. The sensing head includes a light source (LED) and a light sensor (photo detector) to read the generated code (the encoder output).

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