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?
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"?
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.