An expansion is a large addition to a released game that adds substantial new content, such as a new region, campaign, systems, or character options, and requires the base game. Expansions are bigger in scope than typical downloadable content packs.
Before downloads, expansions arrived on disc and were a PC institution. Warcraft II Beyond the Dark Portal, Diablo II Lord of Destruction, and The Sims expansions added campaigns, classes, items, and systems significant enough to extend a game life by years. An expansion was expected to feel like a smaller sequel built on the same foundation, priced below a full game and aimed at players who had finished the original.
The form survived the move online intact. The Witcher 3 Blood and Wine added a full region and a main quest line long enough to review like a standalone RPG. Final Fantasy XIV expansions function as full sequel scale chapters, raising level caps and adding jobs, zones, and dozens of hours of story, and they anchor the game release calendar every few years.
Players and publishers draw the line against ordinary DLC by substance, not file size. A costume pack is DLC. A new landmass with its own story, enemies, and progression is an expansion, whatever the store page calls it, and it is priced and judged accordingly.
In play"Wait for the expansion if you want a whole new region, not just a few extra missions."