You can't expect a book called Level Up! not to have a chapter on level design. But what do I mean by level? Much like the word score, its definition changes when used in different contexts. Observe:
Level: an environment or location where game play occurs. "If you are on the Death Star level then you are near the end of the game."
Level: a term favored by developers that describes how to break up physical space based on a specific gameplay experience. "I must have died a dozen times on the mine cart level."
Level: a unit of counting the player's progression, especially, when repetitive gameplay is involved. "I'm up to level 20 on Tetris."
Level: the rank of a player based on their earned score, experience, or skills. A term for marking character progression and improvement as in: "I finally leveled up my third WoW character to 70." The most common use of level in this context can be found in RPGs.
A possible explanation for level having four definitions in the video game industry is that video game developers have extremely limited vocabularies.
Another reason for the multiple definitions is that developers have used the term "level" in different contexts for so long that it's too late to get everyone to agree to call it something else like a floobit or a placenheimer. So level it is. But why level? Most ancient game designers believe that the term came from Dungeons and Dragons, when players would ...