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 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"?