Plaid is a pattern of horizontal and vertical bands in two or more colors that cross regularly.
The most obvious example is tartan, the classic Scottish pattern; gingham and Border tartans are a variant that has equal-width bands of colour in a simpler pattern. Tattersall has very wide bands alternating with very narrow bands, again in contrasting colours.
In contemporary menswear terms, however, refers to three individual fabric pattern styles: windowpane, glen plaid and Prince of Wales. Windowpane checks are very open box-like patterns using thin pinstripes of colour, normally no more than one or two. The glen plaid is a tight pattern of broken checks. The Prince of Wales check is a subtle combination of the other two, a windowpane check overlaid on a glen plaid.
You will find men’s suits in all of those. Madras is the other kind of plaid, a pattern that is featured on lightweight cotton (often called Madras cloth) and frequently used for casual shirts.
Plaid suits are considered by some as inappropriate for business wear and for formal occasions, but modern styling and contemporary designs are changing that perception. A man’s suit in a plaid can be even as figure-enhancing as a solid color or a muted pattern, and the fact that there is a pattern at all instantly reduces the formality of the suit.