concurrent

Short definition from Wiktionary

Adjective

1. 1
   Happening at the same time; simultaneous.
2. 2
   Belonging to the same period; contemporary.
3. 3
   Acting in conjunction; agreeing in the same act or opinion; contributing to the same event or effect.
4. 4
   Joint and equal in authority; taking cognizance of similar questions; operating on the same objects.
5. 5
   Meeting in one point.
6. 6
   Running alongside one another on parallel courses; moving together in space.
7. 7
   Designed to run independently, rather than sequentially, using various mechanisms, such as threads, event loops or time-slicing.

Noun

1. 1
   One who, or that which, concurs; a joint or contributory cause.
2. 2
   One pursuing the same course, or seeking the same objects; hence, a rival; an opponent.
3. 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. 4
   One who accompanies a sheriff's officer as witness.