What is a Tartan?
Tartan refers to the pattern of interlocking stripes, running in both the warp and weft in the cloth (horizontal and vertical), or any representation of such a woven design in other media (printed, painted, or otherwise rendered). Typically today, one thinks of "clan tartans" -- that is, tartan designs that represent certain Scottish clans and families. While this is typical, it was not always so. While tartan-like textiles have been around since the 3rd century AD, it is generally regarded that tartans associated with Scottish clans, families or institutions date no earlier than the mid-nineteenth century.
In many countries today, the pattern of interlocking stripes called a tartan is often mistakenly known as "plaid." Plaide actually comes from the Gaelic word for a blanket, and is specifically used in the context of Highland dress to refer to a large length of material. The original kilt was known as the "belted plaid" and consisted of a length of cloth (basically a large blanket) that was gathered and belted at the waist. The plaids were most often made from a tartan cloth, and so the confusion between the two terms is understandable.