Term: Threat Level / TL

Threat Level, TL, or as it used to be called 'Challenge Rating', is way of summarizing the amount of danger an Encounter may have. For a party of Player Characters, their Threat Level equals all of the character's levels combined, plus the Item Power Level and Magic Power Level of the characters' items.
For Nonplayer Characters, hazards, or objects, they have a base TL that adds to the rating, making an encounter deadlier or last longer based on certain factors. This can also help scale the opposing creature appropriately, using resources outside the game or the 'Scaling and Alterations' section on the Creature's Index Page. When comparing NPC TL to Party TL, a good rule of thumb is to multiply the total encounter's TL by a factor of 4, trying to keep the numbers as close as possible.
All told, circumstances and the number of player characters can change how things will actually go in the encounter, so adjustments have to be made by the GM if not following a premade Adventure.