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:

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

ary = [] of RecArray
ary.push [Gray Blaze Under Men's 3 Phoenix Fire Orange Speedform Armour Glacier Gemini Shoe Running 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Brown Giant Boot Men's Wellington Georgia Georgia Work Boot wxO7T0Rntq

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