1. A system or arrangement in which one must actively choose to participate
   "The company uses an opt-in policy for its marketing emails"
    
2. The act of choosing to participate in something
   "The opt-in for the new program was higher than expected"

1. Choose to participate in something, especially a program or system
   "Many employees opted in to the new retirement plan"

Derived forms: opting in, opted in, opts in

Encyclopedia: Opt-in