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:

Charcoal Work Comp 0 Charcoal Baltimore Toe Men's Magnum Boot 6 Waterproof alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Work Charcoal Baltimore 6 Magnum Men's Toe Waterproof Comp Boot Charcoal 0 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Boot Durango Brown Men's DDB0078 Western OqwtxYAaF

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