Sanskrit दुःख dukha, Pali dukka, Tibetan སྡུག་བསྔལ་ sdug bsngal, pr. "duk-ngel": the most basic interpretation of this important word is "unsatisfactoriness."
In Buddhism, the Four Noble Truths explain the basis of spiritual pursuit:
"It is through not understanding, not realizing four things, that I, Disciples, as well as you, had to wander so long through this round of rebirths. And what are these four things? They are the Noble Truth of Suffering, the Noble Truth of the Origin of Suffering, the Noble Truth of the Extinction of Suffering, the Noble Truth of the Path that leads to the Extinction of Suffering."
It is said that there are three types of suffering:
Suffering of suffering: pain, birth, illness, growing old, and dying
Suffering of change: the anxiety or stress of trying to hold on to things that are constantly changing
Suffering of conditioned existence: unsatisfactoriness, caused by mistaken perception of “self”
“Nirvana” [literally "cessation"] or liberation is the release from dukha, suffering.
"Only a tremendous revolution can save us from the abyss. When the human being dissolves the “I,” then there is a complete revolution. The human being will stop suffering only when he will be capable of dissolving the “I,” since pain is nothing more than the result of our evil deeds. Thus, pain belongs to Satan (the “I”) because he is the one who commits the evil deeds. The Absolute Abstract Space, the Universal Spirit of Life, is absolute happiness, supreme peace and abundance. Therefore, those who make a mysticism out of pain are masochists, since Satan (the “I”) was and is the creator of pain." - The Aquarian Message
“You are liberated when your delusions and contaminated karmic actions are exhausted.” - Nagarjuna
"The personality, the individuality and the “I” are the hard chains that bind us to the hard rock of suffering and bitterness. Gods and humans are submitted to the suffering of conditioned life." - The Major Mysteries