(1) Happening at the same time as something else; operating or acting in conjunction with another (Citation)
(2) Meeting or tending to meet at the same point; being in accordance; harmonious. (Citation)
(3) Concurrent engineering is a work methodology based on the parallelization of tasks (i.e. performing tasks concurrently). It refers to an approach used in product development in which functions of design engineering, manufacturing engineering and other functions are integrated to reduce the elapsed time required to bring a new product to the market. (Citation)
(4) A concurrent life cycle approach applies lifecycle activities in parallel to ensure the necessary relationships between them are considered within the lifecycle (Citation)
(1) & (2) Oxford English Dictionary
(3) Ma, Y., Chen, G. & Thimm, G.; "Paradigm Shift: Unified and Associative Feature-based Concurrent Engineering and Collaborative Engineering", Journal of Intelligent Manufacturing, DOI 10.1007/s10845-008-0128-y
(4) cited reference needed
(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 prduction world, where the idea of concurrency is used to both shorten delivery time and improv quality by putting activites in parallel which need to work closely together.
(4) is the more general lifecycle concept building on these ideas. (reference needed, possibly from B Boehm spiral or ICM reference)