Definition:Predicate

The predicate of a simple statement in logic is the part of the statement which defines what is being said about the subject.

The predicates of a simple statements are atomic in predicate logic.

The subject and predicate of a simple statement are referred to as its terms.

The "Is" of Predication
Consider the statement: "Socrates is a man."

This means "The object named Socrates has the property of being a man."

Thus we see that "is" here means "has the property of being".

In this context, "is" here is called the "is" of predication.

The "Is" of Identity
Compare this with the sentence "Socrates is the philosopher who taught Plato."

We could of course reword this as: "The object named Socrates has the property of being the philosopher who taught Plato."

However, the meaning that is really being conveyed here is that of "The object named Socrates is the same object as the object which is the philosopher who taught Plato."

In this circumstance, "is" is not treated in the same way as the "is" of predication.

When being used to indicate that one object is the same object as another object, "is" is called the "is" of identity.

In this context, "is" means the same as equals.

Quote
"It all depends on what you mean by 'is'." -- W.J. Clinton