What is an 'endpoint' in Kubernetes?

While you're correct that in the glossary there's indeed no entry for endpoint, it is a well defined Kubernetes network concept or abstraction. Since it's of secondary nature, you'd usually not directly manipulate it. There's a core resource Endpoint defined and it's also supported on the command line:

$ kubectl get endpoints
NAME         ENDPOINTS            AGE
kubernetes   192.168.64.13:8443   10d

And there you see what it effectively is: an IP address and a port. Usually, you'd let a service manage endpoints (one EP per pod the service routes traffic to) but you can also manually manage them if you have a use case that requires it.

Pods expose themselves through endpoints to a service. It is if you will part of a pod.

Source: Services and Endpoints