In general, mathematically, input refers to an element of the Domain of a Function.

In Supervised learning, an input refers to an input of the function the learning algorithm outputs. That is an input is an element of the {input space which the algorithm is learning to map to an output space (a label in the case of classification)}.

In Engineering, particularly Electronic engineering and Signal processing, input refers to either Signals fed into a certain engineering system, or this part of the system itself, whose full name is Input device.