In a discussion about domain driven design I have learned the different people seem to think of different things when using the word aggregate. The main difficulty is that some people use the word aggregate for what other people call aggregate type.
It is quite difficult to have a discussion if people assume different meaning for the same words. For this reason I set out trying to clarify on what most people and the literature agrees too. If you give an answer to this question I would be very happy if you could provide a reference to literature.
For one person an aggregate is the boundary that groups a collection of entities. It is more a conceptional clustering boundary.
For another person an aggregate is a collection of entities transfered from a database repository (having transitional consistency). So an aggregate is something real and not just a concept. If I for example load two users from a database then I have loaded two aggregates of the same aggregate type.
Another person that also thinks that a collection of entities that are transactional consent but thinks that if you load data of a given aggregate type you can also load it partially (with some data just null for example) and still call the whole thing one aggregate while others would see this as two aggregates (with eventual consistency, meaning the consistency is given after both aggregates are saved).
To find the true meaning of the word aggregate myself I have had a look at the definition of Martin Fowler. Here an aggregate is something real and there can be two aggregates of the same aggregate type. But when reading something like this article from Vaughn Vernon I get the impression that he calls aggregate what according to the 'Martin Folwer like interpreted understanding' should be called aggregate type.