Concurrent Users
Concurrent users is the number of users actively interacting with a system at the same moment — the key input variable in load and performance testing, since it's simultaneous demand, not total registered users, that actually determines whether a system holds up under real traffic.
A system with a million registered users but only a hundred active at any given moment has very different performance needs than one with a hundred thousand users all logged in and actively transacting at once — concurrent users, not total users, is the number that maps to actual server load.
Load testing tools simulate concurrent users as virtual users — scripted sessions running in parallel against the system, ramped up gradually rather than launched all at once, to model realistic traffic growth rather than an unrealistic instant spike.
