Here’s a great post from Eric Gunnerson’s blog on some new terminology we’ll see with generics.
I've been diving into generics a bit more deeply, and I'd like to share some of the new terminology that will be floating around.
An open type is a generic type that you've defined. List<T> is a good example. It's called "open" because it's not fully defined without knowing what T is, and therefore you can never have an instance of this type.
The type placeholder that you use when writing a generic type. In List<T>, T is the type parameter.
A constructed type is what you get when you specify a type for a type parameter. List<int> is a constructed type, and you can create instances of it.
The actual type you use instead of the type parameter when creating a construct type . In List<int>, int is the type argument.
A method that has a type parameter on it.
static int Process<T>(T t);
is an example
Strictly speaking, the number of parameters to a method. In generic terms, it's the number of type parameters on a generic type or method. ArrayList has an arity of zero, while List<T> has an arity of one.
[Eric Gunnerson's C# Compendium]