The vocative case (abbreviated voc) is the case used for a noun identifying the person (animal, object, etc.) being addressed. For this reason, they are sometimes referred to as “nouns of direct address”. Vocatives are always second person and they are parsed almost exactly like nouns in the nominative case (BBG 13.10).
In the sentence,
Jeb yelled, “I don’t know, John, we might have to leap overboard.”
“John” is in the vocative case. In the NT, it is usually singular and ends in –ε (e.g. ανθρωπε).