Enlistment (noun)

1. The process of signing up to serve in the military or to join a particular organization or cause.
2. The state or period of being enrolled and serving in the military.

Origin:

From the english word "enlist" which comes from the old french word enlistier, which means "to enroll".

Examples:

1. He completed his enlistment in the army after 4 years of service.
2. The enlistment process includes a medical examination and background check.
3. He decided to postpone his college enrollment to complete his enlistment first.
4. The enlistment age for the military is 18 years old.
5. The enlistment numbers are lower than expected this year.