An undesirable state of being, which can be but isn't necessarily convergent with pain. For example, the muscle pain from an intense work-out wouldn't be called "suffering" by most people, but intense depression would, despite not being painful. This is, incidentally, how most people use the word.

Either way, it's definitely not "pain so intense and prolonged that it makes people avoid it at all costs, including suicide", because in that case, the term "moderate suffering" would be a self-contradiction, and that's just absurd and definitely not common-sensical.

If you're genuinely curious about the question of defining suffering, I can elaborate or offer some reading recommendations, but otherwise, I'd like to go back to topic, which is that existential confusion/dread/suffering (call it as you will, the term is secondary in its importance) is relevant to the majority of mankind regardless of living conditions or physical state, thus your initial claim that the problem is irrelevant to most still fails.