The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
272. The Predicate of a sentence may be a Verb (as in canis currit, the dog runs), or it
may consist of some form of sum and
Noun or Adjective which describes or defines the subject (as in Caesar consul erat, Caesar was consul).
Such a noun or adjective is called a Predicate Noun or Adjective, and
the verb sum is called the Copula (i.e. the connective).
Thus in the example given, Caesar
is the subject, consul the predicate
noun, and erat the copula (see § 283).