CrossFit is many things..
Mainly, it is a fitness regimen developed by Coach Greg Classman over several decades.
Glassman was the first person in history to define fitness in both a meaningful and measurable way.
CrossFit is defined as that which optimises fitness – constantly varied functional movements performed at relatively high intensity. CrossFit also refers to the communal aspect of athletes who do train together and is a key component of why it is so effective.