One pursuing the same course, or seeking the same objects; hence, a rival; an opponent.
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.
concurrency
The property or an instance of being concurrent; something that happens at the sametime as something else.
(computer science, by extension) A property of systems where several processes execute at the same time.