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3e used to have a "delay" option where you could opt to move down the initiative order (and stay there going forward), but 5e did away with it and the only way to change when you act is via the Ready action.

The way readying usually works is basically "move + guard", though it's more flexible than that in regular D&D with a human DM where you can line up whatever action you want like "I'll stay put, but if the goblin comes toward me I retreat into the next room" rather than only being for attacks.

But if they wanted to only implement it as letting you attack or cast a spell when an enemy enters a target area, that would be a lot better than nothing.