Durometer
Measure of the hardness of wheel urethane. The A scale runs from 78A (soft) to 101A (very hard).
Extended definition
78-87A: cruising. 88-95A: park, transitions. 95-99A: hard street/park. 99-101A: technical street. Some brands use the B scale (Bones SPF). 84B ≈ 104A.