Constants, Variables, and Data Types
In OOP languages, an immutable object is an object whose state can’t be modified. Swift calls these constants. Once a constant object has been created and an initial value assigned, it can’t be changed. In contrast, an object whose value can be changed is a mutable object. Swift calls these variables.
In Swift, variables must be defined and have a valid value before they can be used. Use the var keyword to define a variable and the let keyword to define a constant. Note that all types can be defined either as mutable or immutable (constant or variable) based on whether they’re defined with let or var. The language doesn’t require separate mutable and immutable types:
let numberOfWheelsForBicycle ...