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:

Boot Black Men's UGG Chukka Hendrickson Leather alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Black Men's Chukka UGG Hendrickson Leather Boot 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Camo Women Fatbaby Women's Collection Vintage Western Cowboy Boot Ariat Pink Bomber wxvq1d6fp

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