A Guardian book review described how Lurpak butter was named after the lur, a curved brass horn popular in the first and second millennia BC in and around Denmark, referring to this as a "fascinating factoid". It may have been fascinating, but a factoid is not a small fact. It's a mistaken assumption repeated so often that it is believed to be true.

At least, that was the meaning ascribed to the word by Norman Mailer, who is widely credited with coining it, in his 1973 biography of Marilyn Monroe. Mailer said factoids were "facts which have no existence before appearing in a magazine or newspaper".

You can also use factoid as an adjective, to mean "quasi-factual", according to the Oxford English Dictionary, which adds that it is used to designate "writing (esp. journalism) which contains a mixture of fact and supposition or invention presented as accepted fact". I like that "(esp. journalism)".

A true factoid should sound credible, and be assumed to be true by a significant number of people (if you are the only person who believes it, it may simply be a delusion). The Washington Times defined a factoid as "something that looks like a fact, could be a fact, but in fact is not a fact". An example is the belief that the Great Wall of China is visible from the moon, which according to Wikipedia would be possible only if your eyesight were 17,000 times better than 20/20.

Books about grammar and language are full of factoids, such as George Bernard Shaw spelling "fish" as "ghoti" and Winston Churchill writing "this is something up with which I will not put". I think the writers want these stories to be true, so repeat them, even when there is little or no evidence for them.

Wikipedia's entry on factoids links to a long, fascinating list of common misconceptions that may shatter many of your illusions: Napoleon was quite tall, for example, and "Ich bin ein Berliner" really does mean "I am a Berliner" in standard German. Urban legends are also related to factoids in that many people believe them to be true, although in this case they have been told something by other people rather than read it in the newspaper.

As with literally, which currently has two meanings ("literally" and "figuratively"), there is some confusion about what you mean when you use the word "factoid" because a second, albeit related, meaning has crept in – and perhaps even supplanted the original.

In the UK at least, you can blame Steve Wright for this. The DJ has popularised the use of factoid to mean an arcane or trivial, but (sometimes) fascinating, fact through his BBC Radio 2 show and a related book and Twitter feed. There is some overlap between the two meanings: Wright's "a cough can travel at 600mph" may well be true; but "women spend 47 minutes a day getting primped and polished" sounds like a factoid in the Mailer sense; and "in the Peruvian language there are 1,000 words for potato" seems highly unlikely. (The Peruvian language may well contain 1,000 different ways to describe potatoes, but then so does the English language.)

Oxford Dictionaries Online suggests that the more recent use of factoid to mean "a brief or trivial item of news or information" originated in North America. The US writer William Safire felt a new word was needed to differentiate between the two meanings, proposing "factlet" to mean small fact or "little bit of arcana". A Guardian colleague has put forward "fictoid" or "fictlet". But it is probably too late.

We will just have to live with both definitions for as long as there are people who, like me, think the original meaning of factoid is still worth fighting for. It is subtle and useful, conveying so much more than "misconception", and has no ready equivalent.

Besides, there is a more serious side to this. Part of the Mailer definition that is usually overlooked is that the media create factoids not so much as lies but "as a product to manipulate emotion in the Silent Majority". For a sense of what he might have had in mind, consider the "facts" many people believe to be true about the scale of immigration to the UK, say, or the proportion of people claiming benefits.

Perhaps we all have a pet "fact" that we dispute. In my case, it's the claim that generations of children have literally hidden behind the sofa when Doctor Who is on television. This has been repeated so often in the media that it seems eccentric even to question it – but I have been watching the show on and off since the very first episode, when I was 10, and have never hidden once; nor have my children, or anyone I know. It's a factoid.

If you have factoids of your own, do share them below.