"Back-pay"

Have anyone of you heard of this word "back-pay"? It's suppose to be the opposite of "pre-pay". I learnt of this interesting word from my present company. Due to the incapability of the Admin Department's Head, they will announce the promotion of staff late and "back-pay" their pay; at the same time, they will also announce that the staff has been promoted with effective from a date which was earlier than the announcement date. So, it's like a "back-pay" title too. So to the staff who got promoted, he/she would be thinking "damn, I was already promoted 2 weeks ago and I didn't know it!" This is really idiotic.