Dept. is only the written short form of department. You use it on signs, forms, envelopes, and office names to save space: Sales Dept., History Dept. Out loud you still say “department”; you do not say “dept.” Though actually, some writers now drop the full stop and write Dept — same word, same job. It is not a new idea; it is just department in a smaller coat.

部門（書寫縮寫；口頭仍說 department。公司裡是「部／部門」，大學裡常對應「系」）