Wikipedia
Openweight is an unofficial weight class in combat sports. It refers to bouts where there is no weight limit and fighters with a dramatic difference in size can compete against each other. It is different from catch weight, where competitors agree to weigh in at a certain amount without an official weight class.
Judo has held the World Judo Open Championships where judoka from different weight classes can compete, whereas the ADCC Submission Wrestling World Championship holds tournaments every two years with both official weight classes and an "absolute" (no weight) division.
The sport of mixed martial arts originally had no weight classes, and martial artists of different styles and sizes were drawn together to prove which martial art is most effective in unarmed situations. By the early 2000s, weight classes were introduced and became mandatory in many countries. However, Japan became a bastion of openweight fights, with fighters such as Ikuhisa Minowa and Genki Sudo commonly facing much larger opponents.