What is the definition of predicate?

A predicate noun (more correctly called a predicative noun) is a type of complement. The complement element of a clause adds meaning to that of another clause element - either the subject (the subject complement), or the object (the object complement). A subject complement (Cs) renames the subject, for example in 'John is an accountant', 'John' is the subject and 'an accountant' is a subject complement (predicative noun). An object complement (Co) renames the object, for example in 'I find your children angels', 'children' is the object and 'angels' is an object complement (predicative noun). Be careful to avoid confusing 'predicative nouns' with 'predicative adjectives' - the latter describes rather than renames the subject or object. In the above examples if you replace 'an accountant' and 'angels' with 'fat' and 'charming' respectively, these would be predicative adjectives.