Accompaniment; especially; a conjunction that is regular and is marked by correlative variation of accompanying elements

Concurrence.
The term concurrency control refers to sharing resources among multiple users simultaneously.

The quality of belonging to the same period of time; SYN. contemporaneousness.

Happening or existing or done at the same time; SYN. simultaneousness.

ETYM Greek, from, to be contemporary with, from syn with + kronos time. Related to Synchronous.
The relation that exists when things occur at the same time; SYN. synchrony, synchronicity, synchroneity, synchronization, synchronisation, synchronizing.