The acronym AWOL means "absent without leave." It is a military classification for any soldier who is absent from duty without permission. It can result in a loss of pay for the time away from the soldier's assigned post.
A soldier can be charged with being AWOL for being away from the place of duty without requesting permission or if he or she has requested time away and has been denied. Soldiers who are absent for medical reasons must provide official documentation to their superior officers upon their return, or they may be charged with being AWOL. Finally, even if soldiers are present but not at their post, they may be charged. Although this charge does not necessarily lead to a disciplinary action on its own, it may be grounds for future disciplinary action, so it is a serious offense.
This acronym has been in use since at least World War II. In popular slang, AWOL is often used in non-military situations to mean someone is missing without any explanation.