Level
In construction, level (as an adjective) is to horizontal what plumb is to vertical. A level (as a noun) is a device used to measure and obtain a leveled surface, such as flooring or countertop, so that a ball will not roll around.
In a building, a level is equivalent to a story or floor.
In computer and video games, a level (sometimes called stage or map) is a separate area in a game's virtual world, in modern games typically representing a specific location such as a building or a city. In very early games, levels were simply "levels of difficulty", but since areas with few exceptions are ordered by increasing difficulty within a game, levels were directly associated with areas, and the word was quickly adopted for referring to areas.
Each level has an associated mission which may be as simple as walking from point A to point B or as complex as finding several hidden items in a limited time. When the mission is completed, the player usually moves on to the next level; if it is failed, the player must usually try again. Not all games order the levels in a linear sequence; some games allow the player to re-visit levels or choose in which order to complete them.
A person who creates levels for a game is a level designer. The computer programs used for creating levels are called level editors. Designing levels is a complex art that requires care to visual appearance as well as care to gameplay.