saudade

"The famous saudade of the Portuguese is a vague and constant desire for something that does not and probably cannot exist, for something other than the present, a turning towards the past or towards the future; not an active discontent or poignant sadness but an indolent dreaming wistfulness" -In Portugal  (A. F. G. Bell 1912)

Saudade is a Portuguese word with no english equivalent, but pertains to a deep emotional state of nostalgic longing or yearning. Feelings and experiences that once brought happiness or well-being are mixed with a repressed knowledge that the object of these emotions will never return. This is typically directed towards past lovers, deceased family members, one's homeland, or lost youth. Recollecting such things makes one feel alive but simultaneously empty. It generally relates to the loss of something loved and is considered just as consuming as love, although mostly less intense. One can feel saudade even towards the mere feeling of love when it has faded, the emotion itself being what one longs for.
The word saudade has also been used to convey a national feeling. During the Great Portuguese Discoveries (1415 - 1587), many ships were lost as Portugal expanded it's colonial influence in South America, Africa, and Southern Asia. The casualties were great enough to create a sense of saudade across the country. Saudade is also associated with Portugal's decline as a world power. Portugal was once one of the richest monarchies in Europe but during the Golden Age of European Colonialism, it was unable to compete with other european powers. Portugal fell in to a prolonged period of failing which still persists today. Saudade became so Portuguese it is now a Portuguese way of life, often being romanticized in the subject of paintings, music, and writing. 
Saudade first appears in literature during the 13th century in various Portuguese poetries. It originates from the latin solitas meaning loneliness or solitude. Although saudade can be related to the Portuguese condition as a nation, it is still an introverting state of mind. The core feeling of saudade and loneliness are also consistent, allowing them to co-exist when saudade is felt towards other people when one is alone. Separately, both can also create similar physical symptoms in the heart and throat, however with saudade having an element of pull or yearning and solitas being slightly more stressed.

Saudade by 

Almeida Júnior (1899)

Sources

Saudade — Wikipedia, the free encyclopedia. November 16, 2012.
http://en.wikipedia.org/wiki/Saudade

solitas — EUdict. November 16, 2012.
http://www.eudict.com/?lang=lateng&word=solitas