If you declare that Fridays are Pajama Day, did you create a regulation? No. A regulation is a rule put in place by some authority, such as a government. In fact, there might already be a regulation on the books: No pajamas in school.
If you're trying to remember what a regulation is, think about the phrase "rules and regulations." Regulations are often set by governing bodies of a sport or other activity — thus, there is an accepted regulation-size soccer ball, set by FIFA, professional soccer's governing body. Many government agencies are in charge of "regulating" industries, setting up regulations that companies need to follow to keep us all safe.