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:

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

ary = [] of RecArray
ary.push [Magnum Boot Toe Waterproof 6 0 Comp Charcoal Men's Work Baltimore Charcoal 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Wood Wood Dark Pikoi Sandals Men's OLUKAI Dark 6wApqxg

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