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:

2 Men's Players Baseball Trainer Shoes Black Turf Red Mizuno MIZD9 alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Men's Players Black Shoes Red Mizuno Turf Trainer MIZD9 2 Baseball 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Black Samba Mens ADV adidas Skateboarding Gum White RqIESCw

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