From my brief overview of "at-will" contracts, it appears that any contract with a precisely defined period of employment, that is clearly stated, is not an "at-will" contract. My contract had both a beginning and end date both were in writing. The duties were specified, as was the pay.
Employment contracts which do not clearly and unequivocally define the employment period are "at-will" and allow both parties to terminate their responsibilities without penalty. I think this is why most non-government jobs in the U.S. are considered "at-will" - because there is really no defined employment term.

