An apostrophe erroneously inserted before the final "s" in the plural form of a word. Also: greengrocer's apostrophe.
And usual big business policy is counter-balanced by the widespread use of the greengrocers' apostrophe, which sneaks into everything from apple's to yam's. One of our local pubs recently ran a pool tournament and invited "lad's and dad's, mum's and son's, uncle's and niece's, aunt's and nephew's" which may be a record.
Matthew Engel, "I demand an end to the apostrophe: Then the maligned greengrocer will be as literate as you and I," The Guardian, June 6, 2000
But the trouble really starts with the plurals. Although the apostrophe is only meant to be used with initialisms (like VIP) or acronyms (like NGO for non-government organisation), there is now a strong and growing tendency to use the apostrophe to make the plural of ordinary nouns: "Granny Smith Apple's $1.60/kilo"; "Hot pie's".
This common misuse has been affectionately dubbed "the greengrocer's apostrophe" by specialists in English usage. It isn't really fair to single out greengrocers, since other professions and many individuals also use apostrophes in this way. But the greengrocer's apostrophe is commonest in handwritten signs and greengrocers are prominent among those who often have to write quick, informal notices for public display.
The greengrocer's apostrophe is most often found on signs outside shops on goods for sale: "Tool's reduced"; in shop windows: "Sales assistant's wanted"; and on public noticeboards, especially in personal advertisements: "Lawn's mowed at competitive rates".
Sussex R., "QNP," Courier Mail (Queensland, Australia), July 25, 1998
In the nonstandard (`illiterate') use often called in BrE the greengrocer's apostrophe, as in apple's 55P per Ib and We sell the original shepherds pie's (notice in a shop window, Canterbury, England).
Thomas McArthur ed., The Oxford Companion to the English Language, Oxford University Press, September, 1992