Translations seldom work one to one...
"Department" (as in, a department at a university) is translated as "факультет". E.g. "Department of Physics" would be "Фізичний факультет".
"Faculty" in the academic sense is just another branch of the university structure. E.g. One might study at a Department of Physics at the Faculty of Natural Sciences. Ukrainian universities are not organized this way. The Physics and the Biology departments do not belong to a higher structure unit. Therefore, "faculty" is translated as "факультет" as well.