If you’re writing Swift, you will have seen this and most likely simply not known the term “capture list” (which would have been my problem if my headhunter didn’t give me a heads up this was likely to be a question). A Swift closure “closes over” values from the local environment (or context) creating a new execution environment for code inside the closure. Objective-C blocks are also examples of closures. While I usually use “weak” (or “weak self”) in a closure, it’s officially better if you Mark an object as “unowned” if the closure and containing object reference each other and will be destroyed at the same time.