A data type which contains itself. The commonest example is
the list type, in
Haskell:
data List a = Nil | Cons a (List a)
which says a list of a's is either an empty list or a cons cell containing an 'a' (the "head" of the list) and another
list (the "tail").
Recursion is not allowed in
Miranda or Haskell synonym types, so the following
Haskell types are illegal:
type Bad = (Int, Bad)
type Evil = Bool -> Evil
whereas the seeminly equivalent algebraic data types are
acceptable:
data Good = Pair Int Good
data Fine = Fun (Bool->Fine)