Defining what a tartan is is actually harder than one might expect. In common parlance (at least here in the US) we tend to think of any named design as a "tartan" and if the design is not named (and used primarily for fashion purposes) we say "that's just a plaid." However, this common usage does not actually reflect the historic meanings of either word.

It is interesting to note that Gaelic has no actual word for tartan. The one that is used is breacan which actually means "speckled." Plaid is derived from the Gaelic word for "blanket" and really referrs to the garment (i.e. the belted plaid, or a fly plaid, or a piper's plaid) and not to the pattern. It's only because plaids are most often made of a tartan that the two terms have become confused. But it is quite possible to have a solid colored plaid.

The word tartan itself derives from the French word tiretaine which seems to have originally referred to a type of linsey-woolsey cloth (a linen/woolen blend) regardless of any pattern. How the word came to be associated, in Scotland, with a specific pattern of cloth is not known.

The broadest definition of "tartan" would be "cloth woven with a series of stripes in both the warp and the weft, or a graphic representation of such a pattern."

Most of the time when you are talking about traditional tartan design you will have a regularly repeating pattern that is the same in the warp and the weft. When we think of traditional Scottish tartan designs, this is what we think of. But there are exceptions to every rule.

The most notable here would be the new Welsh named tartans being created by the Welsh Tartan Centre. These are unusual in the fact that they have different warp and weft designs entirely. But there are even some traditional Scottish tartans that do this. One that comes to mind is the McKerrell of Hillhouse tartan, which comes from a portrait dated to the 1870s. The pattern is regular except that the red line of the warp is replaced with a yellow line in the weft.
http://www.tartansauthority.com/web/...r=By+Tartan+No.

I've examined some plaids (by which I mean the garment) from the nineteenth century that were hand woven, in Scotland, in unnamed tartan patters for fashion use that had irregular warp & weft.

And some recently designed Scottish tartans, such as Coigach, have a different warp and weft.
http://www.tartansauthority.com/web/...txtTartan=6570

So, while we can safely say that most traditional tartans have the same pattern in warp and weft, and this is the usual characteristic of a tartan, there are exceptions to every rule.

Aye,
Matt