Std 100-1992 "Standard Dictionary of Electrical and Electronic Terms" (Std 100-1992) has many definitions for 'Relay'! Conversely, the Standard's only 'Contactor' definition calls it a 'Load-Switch'! The point is that there is no simple definition.
contactor
relay
Historical Perspective
In 1905 'relay' and 'contactor' were defined by the AIEE (predecessor of today's IEEE) as follows:
Relay
A device... by which contacts in one circuit are operated by a change in conditions in the same circuit or in one or more associated circuits.
Contactor
A device... for repeatedly establishing and interrupting an electric power circuit.
Now, in response to your "difference" question... the terms are delineated by the power they must carry. Here are definitions presented in Struthers-Dunn "Relay Engineering" manual (ca 1945):
Relay
An electrically controlled device that opens and closes electrical contacts to effect the operation of other devices in the same or another circuit.
Contactor
A magnetically-operated device, for repeatedly establishing and interrupting an electrical power circuit. It is usually applied to devices controlling power above 5kW, whereas the term 'relay' is ordinarily employed below 5kW. The terms are often used interchangeably.
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