CR — Challenge Rating
A rough measure of how dangerous a creature or encounter is compared with the player characters.
Challenge Rating (CR) is a guide for the Game Master (GM) when picking monsters. A higher number means a more dangerous creature; fractions like 1/4 are weak ones. It is only an estimate. Numbers, tactics, terrain and luck can make a fight much easier or harder than its rating suggests.
In Fantasy RPG
Fantasy RPG ratings run from 0 and fractions (1/8, 1/4, 1/2) upward. Encounter difficulty (Easy, Moderate, Hard, Deadly) is estimated from the monsters' XP against the party's size and level, and is marked as an estimate.