What exactly is a buildpack in DevOps?

I am familiar with the term buildpack from a platform called Heroku. But I am not sure if it’s a Heroku-specific term, or if it exists more broadly in DevOps nomenclature?

When I google, I see plenty of articles about buildpacks that don’t mention Heroku, so I guess it’s more general. If that’s the case, what exactly is it? And how does it differ from something like a docker image? Or are they the same things with different names?