What triggers 'game-over' in a standard Osu beatmap?

I recently started playing the standard Osu beatmaps - I'm not sure of the technical term of the format but I'm referring to the traditional beatmaps which involve clicks, sliders, reverse sliders and spinners.

My question: What exactly triggers 'game-over' when playing a beatmap?

What I know: if I mess up a lot in a particular run through of a beatmap, it's game over (i.e., the beatmap stops)

What I don't know: What is the exact termination condition for a beatmap (assuming no mods)? In other words, what exactly has to happen to trigger 'game-over' when playing a beat-map?

From observation, it seems that termination is linked to some threshold number of mistakes that can occur within some time-frame.

It also seems as though it's not related to total accuracy since the start of the song. For example, I've had run-throughs where it was game-over after I made a bunch of mistakes one-after-another, but, even after taking these most recent mistakes into account, my cumulative accuracy on that run-through (up until the point of termination) was 85%. Meanwhile, I've completed beatmaps where I've maintained a 70-75% accuracy throughout.

By the 'game over' screen you mean this?

Well, you'll only reach it if you lose all your Health (as in reach 0) without the No Fail mod on.
Other conditions that also triggers it are:

• Hardcore mod - Your health instantly drops to 0 if you miss a beat or break the combo.
• "Pretty much every other mode" - When your health depletes away to 0.^

^ Note that the Perfect (SS or Quit) mod restarts (same as pressing Ctrl-R) the beatmap when you mess up the perfect run (100, miss or combo break).

As for avoiding this, refer to this osu! Wiki quote

• The top left bar is your "health".
  • This will decrease at a steady rate, depending on the beatmap difficulty set by the mapper(s), but you can replenish it by hitting notes at the right time.
  • A perfectly timed hit (300 hit or Geki Beat) will increase your health more than a badly timed hit (50 hit).
  • A total miss will take a good chunk out of your health.

Depending on the difficulty of the beatmap, your perfect (or on-time beats) and misses will affect your score differently.

The drain rate determines the speed your health depletes.
The higher the drain rate, the more accurate you'll have to be to avoid draining faster than your hits can return.

Just to be clear, a 50, 100, 300 will increase your health, while misses, combo breaks will take a chunk out of your health.

Apart from that, messing up (missing) a beat or breaking a combo will remove 1/4-1/3^# of your health bar.

^# It's either 1-third or 1-quarter.

Note: Combos are broken when your combo count (bottom left) is reset to 0.
This happens when you:

• Miss a beat (also counts as a Miss)
• Didn't turn a spinner fast enough (also counts as a Miss)
• Release the key on a slider too early (also counts as a 100)
• Missed the start of a slider but still manage to catch up and drag (also counts as a 100)
• Missed a slider completely; not click/dragged at all (also counts as a Miss)

Your combo is basically a score multiplier and can be increased by:

• Hitting a note on time (scoring a 50, 100 or 300)
• Spinning a Spinner in time
• Starting a slider
• Staying on a slider (without breaking the combo) whilst going over a beat.
• Staying on a slider, hitting the reverse.
• Completing a slider.

(Which is why sliders can create ridiculous combos whilst scoring very little)