Confusingly, both. It depends on what you mean.
En kaffe means a cup of coffee. "Vill du ta en kaffe efter jobbet?" = "Do you want to go grab a coffee after work?"
Ett kaffe refers to the beverage itself, or to a certain type of coffee.
thanks. I'm just kinda confused because DL doesn't always display the articles when introducing new substantives.
En kopp kaffe = a cup of coffee or just "ett kaffe" as mentioned before. Hope that helps as well :)