The OED describes a 'wraith' as a ghost or a ghostlike image of someone. And Tolkien used the word mostly for his Nazgūl, the Ring-wraiths, but occasionally also in different contexts to refer to such ghostlike images of someone.
But in his poem "Mythopoeia", the context and meaning is slightly different:
|Blessed are the men of Noah's race that build|
their little arks, though frail and poorly filled,
and steer through winds contrary towards a wraith,
a rumour of a harbour guessed by faith.
Here 'wraith' is explained as a rumor of a harbor guessed by faith, which can still be considered a ghostlike image, but what throws a somewhat different light at this line from the poem is the fragment
Westra lage wegas rehtas, nu isti sa wraithas,
which in The Lost Road occurs to Alboin in a dream and is translated as 'a straight road lay westward, now it is bent', and thus wraithas here means 'bent'.
Shippey, in JRR Tolkien-Author of the Century (p.287), claims that the language here is Old or Proto-Germanic?. (Lateron, in the "Notion Club Papers", the same fragment appears, but there has wraikwas instead of wraithas.)
Interesting also are Shippeys etymological explanations regarding wraith (ibid. p.122), where he suggests that 'wraith' derives from Old English wrišan, to 'writhe'. Other - related - words are 'wreath', something that is twisted, 'wroth' - an old adjective meaning 'angry' - and the corresponding noun 'wrath', which survived.
Thus Tolkien's concept of wraiths, as represented by the Nazgūl, probably portrays not merely ghosts in the traditional sense, they were also - implicitly - 'twisted' and 'angry'...
See also: Nazgūl