concurrent

1. One who, or that which, concurs; a joint or contributory cause.
2. One pursuing the same course, or seeking the same objects; hence, a rival; an opponent.
3. One of the supernumerary days of the year over fifty-two complete weeks; so called because they concur with the solar cycle, the course of which they follow.
4. One who accompanies a sheriff's officer as witness.

الجمع concurrents