What defines a closure in JavaScript?

A closure in JavaScript is defined as a function that is bundled together with its lexical scope. This means that the function retains access to the variables defined in its outer scope, even after that outer function has finished executing.

When a function is created inside another function, it forms a closure that encompasses its surrounding environment, allowing it to remember the context in which it was created. As a result, closures can be useful for creating private variables or functions that can maintain state across multiple calls.

This behavior distinguishes closures from ordinary functions that are not tied to their lexical environment, allowing closures to access variables that are no longer in scope. Thus, the ability of a function to "capture" and remember its context is what fundamentally characterizes closures in JavaScript.