The word nostalgia is Greek and derived from two words: nostos (νοστος), which means a return, ΄and algos (algos), meaning pain. It is the kind of pain that the ancient Greek hero, Odysseus, felt, when he was detained for many years on the island of Ogygia by the nymph Calypsio, who hd fallen in love with him, until Zeus finally instructed her to let him go. So the word nostalgia, together with its cognate adjective, nostalgic, conveys a longing for something lost, whether in time or space.
Because it a matter of human feelings or emotions, sharp definitions are difficult and may differ between dictionaries. Thus The Cambridge (online) English Dictionary offers:
a feeling of pleasure and slight sadness when you think about things that happened in the past.
an affectionate feeling you have for the past, especially for a particularly happy time.
But this time, it offers synonyms that show there is more to the word than that.
reminiscence, longing, regret, pining...
Here, it becomes obvious that in there is more to it than the actual definition suggests. Moreover, depending on the context, 'nostalgia' can be a pleasant or an unpleasant sensation, and that it can prompt either sympathy or criticism from others.
The Collins English Dictionary, like the CED, offers separate British and US English definitions.
In British English:
- a yearning for the return of past circumstances, events, etc
- the evocation of this emotion, as in a book, film, etc
- longing for home or family; homesickness
In US English
- a longing to go back to one's home, home town, or homeland; homesickness
2. a longing for something far away or long ago or for former happy circumstances
Moreover, both the noun and the adjective can be used to refer to feelings and attitudes that seek to resurrect or return to circumstances or ideas of the past. In literature in either British or US usage it can be either derogatory or positive.