Dukkha

This Pali term is not reducable to a single English word, but "pain", "suffering", "stress", "unsatisfactoriness", and "angst" have been used.

The first of the Buddha's Four Noble Truths (the statement of the fundamentals of Buddhism) states:
All of life involves dukkha.

The other three Noble Truths state the origin of dukkha, that it can be eliminated (the state of Nirvana, Sanskrit form, or nibbana, Pali form), and the Buddha's method for eliminating it, the Noble Eightfold Path).

The Buddha repeatedly stated that the only purpose of his system of thought was the cessation of dukkha, and refused to join in speculation about metaphysical subjects which would not advance this end.

• From the "Access to Insight" site

Section 3.H.ii "The First Truth" of "Wings to Awakening"
http://www.accesstoinsight.org/lib/modern/wings/3h2

"Dukkha" http://www.accesstoinsight.org/ptf/dukkha