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:

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

ary = [] of RecArray
ary.push [Men's 3 Orange Gemini Armour Shoe Under Phoenix Fire Blaze Running Gray Glacier Speedform 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Baffin Baffin Taupe Womens Women's Women's Arnaq Womens Arnaq wUwBHx

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