Unsatisfactoriness, suffering, pain, distress, stress. At the root of all kinds of dukkha is craving, or attachment.
The Buddha taught there are three kinds of dukkha:
1) Dukkha as physical and mental pain from stresses such as sickness, old age, and death
2) Dukkha as distress brought about by impermanence and change
3) Dukkha as existential suffering, inherent in formation, oppressive nature of continuous upkeep