Levels (in Tomb Raider used rather than stages) are parts of a whole game, a certain location. You have to solve certain riddles in one level to come to the next. Each Tomb Raider is devided into many many levels (from 4 in the Add-On Unfinished Business to almost 40 in Last Revelation).
Note, that in the Custom Level scene, the term 'level' has been extended. Sometimes when speaking of a custom level, people are speaking of a series of levels that should rater be called game.