In the context of Haskell "ad-hoc" means ad-hoc polymorphism.

See https://wiki.haskell.org/Polymorphism for details on parametric (ie. unconstrained) vs. ad-hoc (ie. constrained) polymorphism. In short the difference is that ad-hoc is parametric + one or more type class constraints. Eg. in:

  λ> :t fmap
  fmap :: Functor f => (a -> b) -> f a -> f b

the type variables a and b are unconstrained while f is constrained by the Functor type class.