From a textbook point, a contactor is just a relay, and the terms are interchangeable.
However, it's generally accepted practice that a contactor is a specific type of relay, which utilises a bar (I'm sure it has a proper name, but my mind's gone blank!) to bridge two individual contacts. Their main advantage is they're far less likely to weld shut, and due to their inherent design, are cheaper to make larger to handle larger currents.
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What's normally called a relay usually only has a single swinging contact, which if it welds shut (should only ever happen if overloaded) will remain shut. The swinging contact is more reliable and far cheaper/easier to produce in small sizes, however it's not something that scales to large sizes easily.
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There are off course variations of each to increase safety i.e. force guided for standard relays that provides a monitor for welded contacts, or travel return contacts that monitor a contactor to check the contacts have fully disengaged. And then you can get them in a mind boggling array of contact options.