A letter of resignation would be the letter given to your employer stating that you intend to leave your current position on a specific date, depending on the amount of notice required. (cvtips.com)
It simply means leaving your current position and moving on to a future filled with many exciting opportunities for learning, work, service and adventure. (idaabbott.com)
Additionally, whenever possible you should let the employer know why you are considering leaving your current position. (rogers-resume-help-center.com)