Climb Category in road cycling gives a value to the difficulty of a climb based primarily on it’s length and steepness.
Here are the 4 categories currently in use:
Category 4: Is typically less than 2km long and about 5 percent grade, or up to 5km at a 2-3 percent grade.
Category 3: This can be as short as one mile with a very steep grade, perhaps 10 percent; or as long as six miles with a grade less than 5 percent.
Category 2: Can be as short as 5km at 8 percent, or as long as 15km at 4 percent.
Category 1: Once the highest category, can be anything from 8km at 8 percent to 20km at 5 percent.
hors catégorie (“above category”): This rating is given to exceptionally tough climbs. This could either be a Category 1 whose summit is also the finish of the stage, or one that is more than 10km long with an average grade of at least 7.5 percent, or up to 25km miles long at 6 percent or steeper.”