Well, within the scope of Highland dress, a plaid is a piece of clothing, not a pattern, as it is understood today. A tartan, however, is defined as: "Any of numerous textile patterns consisting of stripes of varying widths and colors crossed at right angles against a solid background, each forming a distinctive design worn by the members of a Scottish clan." There are varying plaids, including the modern fly plaid, the long piper's plaid, and of course, the belted plaid (also referred to as the great kilt). Hope that helps a little.
~Sav