What do you call the large piece of leather worn around the waist like a belt in a kimono? I couldn't find the term for it. Is there a word for this?
You might be referring to a kidney belt or weightlifting belt:
(Image source: Wikipedia)
These are worn by weightlifters and people whose job involves heavy lifting to prevent hernias. They're worn by motorcycle riders as a back support. In my experience weightlifters more often use a leather belt while motorcylcists more often wear a nylon and plastic belt.