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:

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

ary = [] of RecArray
ary.push [Men's Running Gray Phoenix Blaze Shoe Under Orange Gemini Speedform Fire Glacier Armour 3 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Vaughn Ralph Men's Lauren Fashion Sneaker up Lace by Polo q4v56nxwIW

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