Scalability
A system's ability to handle growing amounts of work (more users, more data) without falling over or slowing to a crawl.
Think of it like
Like a restaurant kitchen handling a sudden rush of a hundred customers on a Saturday night without the whole operation grinding to a halt.
Junior or senior?
Junior sounds like
Uses the word 'scale' without a number behind it.
Senior sounds like
Can cite the actual scale — users, requests/sec, data volume — the systems they built handled.
Ask them
“What's the actual scale — users, requests per second, or data volume — the systems you built handled?”
Sounds like real experience
Gives an actual number — requests per second, concurrent users, data volume — and describes what specifically broke or had to change at that scale.
Probe further if
Uses 'scalable' as a description of the system's design intent without ever citing a real number or a specific thing that had to change as load actually grew.