Serverless doesn't have a good definition because it's not a real concept.

At most lambda/workers/functions are really just PaaS, except instead of uploading all of your code, it's just a single file with a single endpoint.

If we really need a new term, then micro-paas or FaaS works fine, although most FaaS vendors are expanding to running any arbitrary container with multiple endpoints, so we're right back to normal (but modern) PaaS at the end.