The U.S. Department of Labor does not give a definition of what a full-time employee is, but the IRS and the Affordable Care Act (aka Obamacare) define it as anyone working 30 or more hours per week, or 130 hours or more per month.
These definitions may not affect you if your company is not considered an Applicable Large Employer (ALE) by the IRS. Generally, companies with fewer than 50 employees are not considered ALEs, but this can vary for a number of reasons.
State laws may also describe full-time for the purposes of receiving specific benefits, and laws such as COBRA may have an effect as well.