This is the 8079th most frequent Spanish word.
nicolás
"Nicolás" is a Spanish given name equivalent to "Nicholas" in English.
Here, 'Nicolás' is used as a personal name in its typical nominative form, indicating the person being talked about.
Nicolás es mi mejor amigo.
Nicolas is my best friend.
In this sentence, 'Nicolás' is used in the possessive sense—referring to the birthday party that is associated with Nicolas.
Te invito a la fiesta de cumpleaños de Nicolás.
I invite you to Nicolas's birthday party.
Within this context, 'Nicolás' acts as the subject of the clause being discussed.