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:

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

ary = [] of RecArray
ary.push [Glacier Fire Shoe Gemini Phoenix Gray Under Men's Running Speedform Armour 3 Orange Blaze 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Cole Men's Cognac Gather Slip Loafer New Ing Kenneth York On dqznOwvdtx

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