Real-word idioms: "See you then."
Textbooks and websites like to give lists of amusing and colorful idioms. Some of them are hardly used at all, and many are not really fundamental parts of the language.
I post about "real-world idioms" when I become conscious of having used or heard one that isn't usually found on lists.
"See you then" means "We've agreed that I will see you at that time." "We've agreed" is understood. The phrase is a shortening of "I will see you then." The word <em>then</em> has a number of different meanings. In "See you then," it means "at that time."
"What time can we get together?"
"Does 2 pm work for you?"
"Sure. OK. See you then!"
(<em>Then</em> can also mean "accordingly" or "therefore." One can invent a conversation,
"Whom will I be seeing, you or Joe?"
"Joe will be out of the office Wednesday, I'll be in."
"OK, I'll be seeing <em>you</em>, then."
However, in the idiomatic phrase "See you then," it means "at that time.")