Flannel and plaid are not the same. Although flannel and plaid often go together, flannel is a fabric; plaid is a pattern. Plaid can appear in any number of fabrics and colors, and flannel can come in a variety of patterns (although, unless you’re looking at flannel sheets or pajamas, plaid is by far the most common pattern on flannel).
Flannel is a warm, comfortable fabric traditionally made of wool, but also commonly made of cotton. Due to its comfort and warmth, flannel is the fabric of choice for much winter apparel. As mentioned above, aside from flannel shirts which are most commonly made with a plaid pattern (or solid color with no pattern), flannel can actually be found in a wide array of patterns.
Plaid, on the other hand, is a pattern. Most believe plaid originated in Scotland, where it’s called tartan. This universally recognized pattern of crisscrossing horizontal and vertical bands of color is popular in both men’s and women’s clothing and accessories, regardless of the type of fabric.
So, bottom line: you can have a plaid flannel, but plaid & flannel are not the same.