A vocative expression is an expression of direct address, wherein
the identity of the party being spoken to is set forth expressly within a
sentence. For example, in the sentence, "I don't know, John.", John is
a vocative expression indicating the party who is being addressed.
Some languages (e.g., Greek) have a special vocative case for this; others do not. English simply uses the subjective case
for vocative expressions but sets them off from the rest of the sentences
with pauses (rendered in writing as commas).
A vocative expression is interjective and can occur in any clause, irrespective of mood. Some examples...