As someone who has done plenty of sound recordist work, it’s known as ‘room tone.’
Also, I feel seen because I’ve had to explain that so many times. Even to people who really should know.
I don’t work in a sound recording space, but I know this because I had to do a voice recording for a project in middle school.
The recording picked up some extraneous noise in between a couple sentences, so I opened Audacity, took the noisy chunk, and deleted it outright so that there was empty space. It sounded very weird for some reason… so I simply recorded over the gap with me doing nothing, as if I were in between sentences. It worked! And I had no idea why…
Granted, this was less ‘room tone’ than ‘white noise from the amount of gain on my shitty mic’, but same concept.