This is the 2253rd most frequent Russian word.
заместителя
"Заместителя" translates to "deputy" or "substitute" in English.
In this sentence, "заместителем" is used in the instrumental case to indicate with whom the action of discussing was performed.
Я обсуждал этот проект с заместителем директора.
I was discussing this project with the deputy director.
Here, "заместителя" is in the genitive case, as it is part of the action of appointing someone to a position, emphasizing the individual being appointed.
Заместителя начальника назначили ответственным за новый отдел.
The deputy head was appointed responsible for the new department.
In this example, "заместителя" is used in the accusative case to indicate the direct object of the verb "ждем" (are waiting).