In general there can be more than one candidate key for a given relation (table). Only one candidate key is "primary", all others are "alternate".

Thanks that makes sense.

Is this an industry term or are you just using it here to emphasize the difference?

I've seen alternate used in other places to refer to candidate keys that are not currently the primary key, though I'm fairly sure it isn't a defined standard term.

Regarding the "industry term", search:

   site:oracle.com "alternate key"

   site:microsoft.com "alternate key"

   site:ibm.com "alternate key"

"defined standard" !== "industry term", though I accept your point that it appears to be more common than I was thinking.

IIRC it wasn't introduced to me when learning databases academically which I was assuming to be a standards compliant environment, but that was quite some years ago so I may or may not R C...

Industry term. But it is usually unknown to people who tend to use id as primary key for every table.

Do you mean synthetic keys rather than natural keys?

Yes, it is exactly a candidate key as they describe it - simply another field which uniquely identifies rows in the entity.

EDIT: Agree with panda17, they're using it as "candidate keys which were not chosen as primary key."