Glossary: exclamative clause
This is a clause type which starts with an exclamative phrase containing what or how, and is typically used to make an exclamation. Examples are What a fuss he made! and How tall she is! The exclamative phrase comes first, so there is a special word order (compare with the usual order in He made a big fuss or She is very tall).
Please note that having an exclamation mark after a clause is not enough to turn it into an exclamative clause (defined as a pattern with what or how), so the following sentence is not an exclamative clause, but a declarative clause, despite the exclamation mark: This is a wonderful event! We see in this last example that it's perfectly fine to add an exclamation mark after clauses that do not have what or how in them; it's just that they are not exclamative clauses, because these are defined as particular structural patterns.
See also: declarative clause, imperative clause, interrogative clause.

