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I have kept the "Check Grammar" option in my browser On, so whenever I write anything wrong as per US English it gets underlined. This is also the case with "multi".

When I use this word in combination with other complete standalone words like multibillion, multimillion or multithreaded, the word "multi" gets underlined (in Red) which means it's not considered correct grammatically.

But it's not considered an error when used in words like multitude, multilateral, etc. Again lateral is a standalone word here but there seems to be no error.

Is "multi" a wrong word in US English? (The said browser software was developed in the USA.)

This link here provides the details about the word "multi" but all of the words used above are considered correct here.

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My browser does not mark multibillion. Maybe yours would prefer multi-billion instead? Just a guess. – GEdgar Jun 30 '13 at 13:05
Multi- is not a word; it’s a prefix. Usually American publishers do not want it hyphenated, although British ones often do. – tchrist Jun 30 '13 at 13:07
The style sheet from O’Reilly says: ‘Unless part of a proper noun, close up words with the prefixes “multi”, “pseudo”, “non”, and “sub” (e.g., “multiuser”, “pseudoattribute”, “nonprogammer”, and “subprocess”).’ However, they make an historical exception for the familiar Unix term pseudo-tty, making an allowance for the accepted practice of writing that word, or perhaps recognizing that tty is an abbreviation that is meant to be spelled out as three letters, not pronounced like tie. It’s not a proper noun, though. – tchrist Jun 30 '13 at 14:36
What do you mean: is it authentic? I assume you don't mean does it comply with the standards set by the U.S. National Bureau of English? But if that's not what you mean, what is your definition of "authentic"? Certainly, I'd trust a style sheet from O'Reilly more than I trust your current browser. – Peter Shor Jun 30 '13 at 14:48
@PHIfounder: there are no standards. The closest thing is Merriam-Webster, which does not hyphenate them. – Peter Shor Jun 30 '13 at 14:57
up vote 4 down vote accepted

You were already given this answer by several of the comments, and it's already implied in polynym's answer, but let's make it explicit:

Yes, the prefix multi is valid in American English, and usually used unhyphenated. You can see dozens of examples on Wiktionary or Merriam-Webster. If your grammar and spelling checker fails to accept it, it should be overridden manually.

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For clarity, I often use a hyphen for some multi- words: multi-threaded rather than multithreaded.

That is accepted by the spelling checker in my current browser where without a hyphen it is underlined in red. There may well be a discussion about appropriate use of the hyphen; I tend to use one as seems right to me and not according to some arbitrary rule.

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Actually, it underlines even in hyphenated form which means it is simply not acceptable. I need to know whether this prefix is valid in US English or not. – 0decimal0 Jun 30 '13 at 14:40
'Using one as seems right to you' is using an arbitrary rule. (Though this one is certainly one I'd have in my arbitrary-rule-box too - where I can't find a dictionary to adjudicate, or where dictionaries offer a choice). – Edwin Ashworth Jun 30 '13 at 15:35

I think context is relevant.

If 'multi' is used in contrast to and alongside another word that is not a valid prefix, then, given the latitude that exists here, I'd opt for the hyphen. So I might write 'I took the multivitamins', but in another context, 'In our experiment, we evaluate both single-threaded and multi-threaded software.'

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