A reverse corporation of sorts, but with only a single source of revenue. A corporation is a collective that works together to produce something. A union, on the other hand, seeks to extract as much value as is possible/reasonable from the host organization, but not so much that they destroy it.
I guess my main points were that corporations differ in that their “host” is basically all of society and they must produce things of value or they die, but a union’s “host” is either a single corporation or industry and they do not directly produce things of value.