This is the 15268th most frequent Russian word.
посетителя
"Visitor" or "guest."
Here, 'посетителя' is in the genitive singular case, indicating possession or association with the visitor.
У посетителя был интересный вопрос.
The visitor had an interesting question.
In this sentence, 'посетителя' is in the accusative singular case because it is the direct object of the verb 'встретил' (met).
Я встретил посетителя музея в фойе.
I met the museum visitor in the lobby.
'Посетителя' is in the genitive singular case, used here because of the preposition 'об' (about/concerning), which governs this case to indicate context of care or attention.