A performer or act, usually a singer or band, who has just one popular success.
'One-hit wonder' is directed at popular musicians, although it is also sometimes used to describe others who are popularly known for a single sucess (unfairly or otherwise); for example, Joseph Heller - referring of course to his best-known work Catch-22. The term is used in various sports too, notably boxing. The meaning there is slightly different from other fields of endeavour. A boxing 'one-hit wonder' is someone who is not considered especially skillful, but relies on single knockout punches to win matches.
In the popular music sense, the term isn't as easy to define as it might seem. Several artists who had several hits in one country might have had just one elsewhere; for example, Frankie Goes To Hollywood is considered a one-hit wonder in the USA, but not in the UK. Also, some artists might be technically one-hit wonders but aren't usually listed as so as they had hits with other bands or under other names - for example, Derek and the Dominoes, Limahl/Kajagoogoo.
this term doesn't appear in print until surprisingly late. The earliest I can find is from the Winnipeg Free Press, in July 1977, in a piece about Abba:
"Instead of becoming what everyone expected [after winning the Eurovision Song Contest with Waterloo], a one-hit wonder, they soon had a string of hits behind them."
The use of the phrase there suggests that it was already known. It would certainly be ironic if the first use 'one-hit wonder' was in an article about Abba - one of the most successful groups in pop history.
There were many one-hit wonders in the 1950s and 1960s, although whether they were called that at the time seems in doubt.
The phrase was used long before 1977 in another context. In baseball a one-hit wonder is a pitcher who restricts the opposing team to a single base-hit by virtue of outstanding pitching. This goes back to at least 1914, as here from the New York newspaper The Middletown Daily Times-Press, July 1914 - in a piece titled One-hit wonder fails to hold locals down:
"The Middies trimmed Newburgh by a score of 6 to 1, although Newburgh were aided and abetted by 'Let-er-go' Gallagher, their one hit pitcher."
Whoever coined the term 'one-hit wonder' in the musical sense might have plucked it out of the air. there's also the possibility that it was an ironic re-use of the earlier phrase - referring to relative failure rather than success. It could also have harked back to an much earlier phrase 'nine days' wonder', which has the same meaning, of something that is a flash in the pan.
In recent years, yet another 'one-hit wonder' has emerged. Anyone who has used Internet search engines will be familiar with searches yielding thousands or millions of hits. A search term that yields just one hit at Google is known as a Googlewhack.