A closure is just a function that captures ("closes over") the surrounding scope.

Edit: strictly speaking, the construct here is an anonymous function. Objective-C calls them "blocks" and Swift calls them "unnamed closures" or "closure expressions." Both are, or at least can be, closures. (I'm not sure if the term "closure" can be used to refer to a function that could refer to variables in the enclosing scope but doesn't. Probably not.)

Blocks are in fact lambdas, or at least closures. I was wrong in the above. In some early versions of smalltalk, blocks had some odd semantics that made them different, but this is no longer true. However, judging by the Objective-C example above, Objective-C blocks don't create true closures, as a true closure would have the semantics of the Swift code from the article.