Can anyone tell me what was meant by the term: "nervous breakdown"

I've been struggling with some episodes of elevated anxiety over the past few days. I get really nervous, my head feels like it's in a vice, & although I can see what's in front of me, somehow it doesn't seem to register in my brain. I've also been experiencing more shakiness in my hands & my legs feel peculiar, almost like they might not hold me up although I know they will... or at least they always have...
Anyway, all of this reminded me of an incident that occurred many years ago. I came in to work one day & one of the two secretaries who worked in the department I worked in was gone. Other staff in the department were talking about the fact that this person had a nervous breakdown at work the day before.

I didn't really understand what this entailed & I still don't.

But it scared the bejeebers out of me because I was a ball of nerves & anxiety all of the time. And if it could happen to her, then I presumed it could happen to me.

(It didn't although I think I've probably been close any number of times over the years since then.)
I presume, nowadays, whatever a nervous breakdown was, it is called by a different name.

I think I may be getting close once again...