Originally a term for a magical force that supposedly resides inside living and inanimate matter. Mana is used in gaming as a magic resource; when characters capable of performing magic cast a spell, magic points (mana) are drained (often, they are automatically replenished over time). In some games, such as the StarCraft series, the term energy is used instead.
determines how easy magic is to use in a particular area. It can vary with geography, time, or activity. In its most common conception mana is viewed as "powering" magic (much like electricity powers modern technology), but it can also be viewed as the "weakness" of reality in a specific area. Some systems a lot different types of mana to different types of mages. Mana ranges through the following: nonexistent, low, standard, high, powerful, and extreme! As there's no cross-genre or cross-system way to measure mana, any estimate that makes sense for your game is appropriate. In GURPS, Mana is both mana rating and added spell fatigue from powerstones and the like. In Storyteller, mana is the inverse of paradox and gauntlet.