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:

Nice Women's André Denim Flat Sandal Assous alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [André Sandal Nice Denim Women's Flat Assous 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Art André Nice Sandal Print Assous Women's Flat xOO6vqX

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