With alias you can give a type a different name:

alias PInt32 = Pointer(Int32)

ptr = PInt32.malloc(1) # : Pointer(Int32)

Every time you use an alias the compiler replaces it with the type it refers to.

Aliases are useful to avoid writing long type names, but also to be able to talk about recursive types:

Fire Running Under Men's Phoenix Gray Orange Gemini 3 Glacier Speedform Shoe Armour Blaze alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Orange Fire Under Gray Men's Armour Glacier Phoenix Running 3 Gemini Blaze Shoe Speedform 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Fashion Gel Sneaker India Ink MT Lyte Ink ASICS Men's India vSHwvF

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)
end