# Is there some definition about risk sharing?

I was searching for a definition of risk sharing and I have found the following:

$$\underline{Definition:}$$ Risk Sharing — also known as "risk distribution," risk sharing means that the premiums and losses of each member of a group of policyholders are allocated within the group based on a predetermined formula. Risk is considered to be shared if there is no policyholder-specific correlation between premiums paid into a captive, for example, and losses paid from the captive's reserve pool.

I am confused with the term "captive". What is the exact meaning of this term? Is there any other reference about seome definition of risk sharing?