Vocative is a term of prayer – word or expression, that highlights the person or thing to whom the word is addressed. It is always separated by a comma.
Vocative is a isolated term within the sentence, that is, it is neither part of the subject nor of the predicate. It is used to call the being you are talking to by name, nickname or characteristic.
The vocative is not classified as an accessory term of the sentence, as it does not maintain any syntactic relationship with the other terms of the clause, since the accessory term may no longer be part of the clause, without prejudice to its understanding.
The vocative can be at the beginning, in the middle or at the end of the sentence. Examples:
o mary, why not clean this house right?
Peter, come here!
Love, I am here.
my friends my enemies, save the country.
Life, my children, must be lived intensely.
orient yourself, boy, by the Cruzeiro do Sul constellation.
What's up, namesake, okay?
when you left me, honey, told me to be happy and have a good time.
This youth is stubborn, your doctor!
See also the meaning of definite and indefinite article.