This is what we found on Wikipedia regarding the World Supercross Championship:

The AMA Supercross Championship (commercially known as Monster Energy AMA Supercross) is an American motorcycle racing series. Founded by the American Motorcyclist Association (AMA) in 1974, the AMA Supercross Championship races are held from January through early May. Supercross is a variant of motocross which involves off-road motorcycles on a constructed dirt track consisting of steep jumps and obstacles; the tracks are usually constructed inside a sports stadium. The easy accessibility and comfort of these stadium venues helped supercross surpass off-road motocross as a spectator attraction in the United States by the late 1970s.

From 1974 until 2002 and again from 2008 until 2021, the series was the World Championship of the sport. After losing this status, and with respect to the MXGP holding that discipline's worldwide title, the series, along with the AMA Motocross Championship, will form the SuperMotocross World Championship from 2023.