A closure is a block of code that you can use as a variable or pass to a function in Swift. Closures are similar to blocks in other programming languages, and they can be used for tasks such as defining a section of code to be executed at a later time or as a callback for a function.

Here is an example of how you can define a closure in Swift:

let greeting = { 
  print("Welcome to Swift Shorts!") 
}

In this example, the greeting constant refers to a closure that simply prints a greeting to the console.

To call the closure, you can use the () operator after the closure’s name, like this:

greeting() // prints "Welcome to Swift Shorts!"

In addition to simply defining blocks of code, closures can also accept parameters and return values. Here is an example of a closure that takes two integers as parameters and returns their sum:

let sum = { (a: Int, b: Int) -> Int in 
  return a + b 
}

To call this closure and use the returned value, you would use the following code:

let result = sum(2, 3) // result is equal to 5

Closures are a powerful feature of Swift, and they are often used in conjunction with other language features, such as higher-order functions and collections. For more information about closures and how to use them, you can refer to the Swift documentation.