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:

Slip Real on amp; OSHOW Shoes House Slippers Clog Lining Outdoor Scuff Indoor Fur Blue alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Clog amp; Outdoor Slip House Fur Real Lining Slippers Scuff Shoes on Blue OSHOW Indoor 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Black Shoe Etnies Red 2 Grey Fader Skate Men's WPqxHrfqY

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