(1) Happening at the same time as something else; operating or acting in conjunction with another. (Oxford English Dictionary Online 2012)
(2) Meeting or tending to meet at the same point; being in accordance; harmonious. (Ma, et al 2008)
(4) A concurrent(Created for SEBoK)
(1) and (2) Oxford English Dictionary. s.v. "Concurrent."
(3) Ma, Y., G. Chen, & G. Thimm. 2008. "Paradigm Shift: Unified and Associative Feature-based Concurrent Engineering and Collaborative Engineering." Journal of Intelligent Manufacturing. DOI 10.1007/s10845-008-0128-y, Springer Science+Business Media
(4) This definition was developed for the SEBoK.
(1) and (2) are dictionary definitions encapsulating the key ideas of things down at the same time, but within some common cause and joint end result.
(3) comes form the production world, where the idea of concurrency is used to both shorten delivery time and improve quality by putting activities in parallel which need to work closely together.
(4) is the more general lifecycle concept building on these ideas.
SEBoK v. 1.9.1 released 16 October 2018