Optional types either contain a value or nil. An optional type is defined with a question mark:

var firstLetter: Character?

Optionals can be assigned to either nil or an instance of the optional type. Its initial value will be nil unless you assign a value yourself:

var firstLetter: Character?
print(firstLetter) // prints nil
firstLetter = “a”
print(firstLetter) // prints Optional("a")

Only optional types can be assigned to nil. If you attempt to assign a non-optional type to nil, you will get a compile-time error.

var firstLetter: Character = “a”
firstLetter = nil // ERROR: nil' cannot be assigned to type 'Character'