"Birth is dukkha, aging is dukkha, death is dukkha; sorrow, lamentation, pain, grief, & despair are dukkha; association with the unbeloved is dukkha; separation from the loved is dukkha; not getting what is wanted is dukkha. In short, the five clinging-aggregates are dukkha."

— SN 56.11

[Ven. Sariputta:] "Now what, friends, is the noble truth of stress? Birth is stressful, aging is stressful, death is stressful; sorrow, lamentation, pain, distress, & despair are stressful; association with the unbeloved is stressful; separation from the loved is stressful; not getting what is wanted is stressful. In short, the five clinging-aggregates are stressful.

"And what is birth? Whatever birth, taking birth, descent, coming-to-be, coming-forth, appearance of aggregates, & acquisition of [sense] spheres of the various beings in this or that group of beings, that is called birth.

"And what is aging? Whatever aging, decrepitude, brokenness, graying, wrinkling, decline of life-force, weakening of the faculties of the various beings in this or that group of beings, that is called aging.

"And what is death? Whatever deceasing, passing away, breaking up, disappearance, dying, death, completion of time, break up of the aggregates, casting off of the body, interruption in the life faculty of the various beings in this or that group of beings, that is called death.

"And what is sorrow? Whatever sorrow, sorrowing, sadness, inward sorrow, inward sadness of anyone suffering from misfortune, touched by a painful thing, that is called sorrow.

"And what is lamentation? Whatever crying, grieving, lamenting, weeping, wailing, lamentation of anyone suffering from misfortune, touched by a painful thing, that is called lamentation.

"And what is pain? Whatever is experienced as bodily pain, bodily discomfort, pain or discomfort born of bodily contact, that is called pain.

"And what is distress? Whatever is experienced as mental pain, mental discomfort, pain or discomfort born of mental contact, that is called distress.

"And what is despair? Whatever despair, despondency, desperation of anyone suffering from misfortune, touched by a painful thing, that is called despair.

"And what is the stress of association with the unbeloved? There is the case where undesirable, unpleasing, unattractive sights, sounds, aromas, flavors, or tactile sensations occur to one; or one has connection, contact, relationship, interaction with those who wish one ill, who wish for one's harm, who wish for one's discomfort, who wish one no security from the yoke. This is called the stress of association with the unbeloved.

"And what is the stress of separation from the loved? There is the case where desirable, pleasing, attractive sights, sounds, aromas, flavors, or tactile sensations do not occur to one; or one has no connection, no contact, no relationship, no interaction with those who wish one well, who wish for one's benefit, who wish for one's comfort, who wish one security from the yoke, nor with one's mother, father, brother, sister, friends, companions, or relatives. This is called the stress of separation from the loved.

"And what is the stress of not getting what is wanted? In beings subject to birth, the wish arises, 'O, may we not be subject to birth, and may birth not come to us.' But this is not to be achieved by wanting. This is the stress of not getting what is wanted. In beings subject to aging... illness... death... sorrow, lamentation, pain, distress, & despair, the wish arises, 'O, may we not be subject to aging... illness... death... sorrow, lamentation, pain, distress, & despair, and may aging... illness... death... sorrow, lamentation, pain, distress, & despair not come to us.' But this is not to be achieved by wanting. This is the stress of not getting what is wanted.

— MN 141

Dukkha is:

Disturbance, irritation, dejection, worry, despair, fear, dread, anguish, anxiety; vulnerability, injury, inability, inferiority; sickness, aging, decay of body and faculties, senility; pain/pleasure; excitement/boredom; deprivation/excess; desire/frustration, suppression; longing/aimlessness; hope/hopelessness; effort, activity, striving/repression; loss, want, insufficiency/satiety; love/lovelessness, friendlessness; dislike, aversion/attraction; parenthood/childlessness; submission/rebellion; decision/indecisiveness, vacillation, uncertainty.

— Francis Story in Suffering, in Vol. II of The Three Basic Facts of Existence (Kandy: Buddhist Publication Society, 1983)

"Both formerly & now, it is only dukkha that I describe, and the cessation of dukkha."

— SN 22.86

"There are these three forms of stressfulness, my friend: the stressfulness of pain, the stressfulness of fabrication, the stressfulness of change. These are the three forms of stressfulness."

...

[Jambukhadika the wanderer:] "What is the path, what is the practice for the full comprehension of these forms of stressfulness?"

"Precisely this Noble Eightfold Path, my friend — right view, right resolve, right speech, right action, right livelihood, right effort, right mindfulness, right concentration. This is the path, this is the practice for the full comprehension of these forms of stressfulness."

— SN 38.14