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For example, is it accurate to say that unac is a prefix of unacceptable, or is un the only valid prefix?

If it is not a prefix is there a word for what unac would be in the above case?

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A prefix is a morpheme, which by definition is not just any set of letters. – RegDwigнt May 2 '12 at 21:02
Any reason for the downvote? – Abe Miessler May 2 '12 at 21:13
Your second question ("Is there a word for this?") is fine IMHO, so my guess would be that the downvotes are due to the first question ("Is a prefix any set of letters...?") being too basic, as you could have easily looked up the answer on Wikipedia, say, or perhaps even just in a dictionary. I wouldn't take that personally (and it's still a net win for you reputation-wise), but perhaps as something to keep in mind in the future. – RegDwigнt May 3 '12 at 9:04
up vote 4 down vote accepted

A prefix is a group of letters that appear at the beginning of a word AND that carry a meaning that modifies the meaning of that word.

So in "unacceptable", "un-" is a prefix that means "not", so "unacceptable" means "not acceptable". Or for example in "antisocial", "anti-" is a prefix meaning "against", so someone who is "antisocial" is "against being social". Etc.

I don't know of any general term for an arbitrary number of beginning letters on a word. Perhaps "leading letters"?

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The word "unacceptable" consists of three parts: un · accept · able.

Un- is the negative prefix, accept is the stem, and -able is the suffix.

So, unac is not a prefix and it is meaningless by itself.

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