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:

Gemini Fire Gray Speedform Orange Running Men's Blaze Armour Under Shoe Glacier 3 Phoenix alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Shoe Gemini Speedform 3 Glacier Blaze Running Men's Orange Fire Gray Phoenix Under Armour 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Dr Martens Canvas Combat Washed Boot Men's Red Combs r6dqAwrx8

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