'Realtime' isn't a specific thing, there's just 'fast enough'. Oldschool "render each frame during the scan line staying ahead of the electron beam" graphics was pretty hardcore though.
> Real-time computing (RTC) is the computer science term for hardware and software systems subject to a "real-time constraint", for example from event to system response.[1] Real-time programs must guarantee response within specified time constraints, often referred to as "deadlines".[2]
Strictly speaking, this definition doesn't say anything about how tight those deadlines are, as long as you can guarantee some deadlines.
There's also 'soft' real time where you try to hit your deadlines often-enough, but there's no guarantees and a missed deadline is not the end of the world. Games are good example of that, including the example of chasing the electron beam.
ABS brakes are an example of a 'hard' real time system: the deadlines aren't nearly as tight as for video game frames, but you really, really can't afford to miss them.