What is a slam attack? - the question is about "Slam" attack name, particularly the Doppelganger's attack:
Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) bludgeoning damage.
The OP asks: "What, descriptively, does an attack of this kind entail?"
I see several problems with this question:
The question implies all "slam" attack are the same, however, a Doppelganger's slam might differ from, say, a Water Elemental slam. It's just an attack title, like "Bite" or "Claw". Besides the generic word meaning, it is impossible to find exact but universal "slam" attack description, thus, answer the question.
The question is about a particular attack title from the MM. I can't understand why only the "slam" was chosen. Why shouldn't we ask "What is a Bite attack?" then, "What is a Claw attack?", "What is a Shortsword attack?", etc.
In 5e MM many attacks aren't actually described, all the descriptive details are up to the DM and, in the end of the day, are opinion-based.
However there were no "primarily opinion-based" VTCs so far and the question has a decent positive score. Why shouldn't it be closed?