From wikipedia:

Dukkha (Pali; Sanskrit duḥkha) is a term found in ancient Indian literature, wherein states Monier-Williams, it means anything that is "uneasy, uncomfortable, unpleasant, difficult, causing pain or sadness". It also refers to a concept in Indian religions about the nature of life that innately includes the "unpleasant", "suffering," "pain," "sorrow", "distress", "grief" or "misery." The term Dukkha does not have a one word English translation, and embodies diverse aspects of unpleasant human experiences. It is opposed to the word sukha, meaning "happiness," "comfort" or "ease."