A tax treatment under which a business is taxed using the Subchapter C laws and regulations. Among other things, this means it is subject to double-taxation. This is the default tax type for corporations, and it is available for election by some LLCs and partnerships.
A C Corp is NOT a particular type of incorporated entity, though it is frequently incorrectly considered to be. Despite the name, C Corps are not always corporations. For instance, an LLC that meets the requirements can choose to be taxed as a C Corp.