Speech turned back and forth in short exchanges. Chat is conversation without ceremony.
Clipped from chatter, Middle English chateren (to twitter, to jabber), of imitative origin, echoing the sound of rapid small talk.
What is missing is the setup, not the substance — a chat needs no agenda, no opening, and no stated purpose to be worth having, and its short exchanges can carry as much weight as a longer, more formal one while asking none of its preparation.