In philately, an invert error occurs when part of a postage stamp is printed upside-down. Inverts are perhaps the most spectacular of a postage stamp errors, not only because of the striking visual appearance, but because they are almost always quite rare, and highly valued by stamp collectors.
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Invert errors, or "inverts" for short, most commonly arise when producing multi-colored stamps via multiple passes through the printing press. It is all too easy for a printing plant worker to insert a half-finished sheet the wrong way around, resulting in the inverts. Such an error being so obvious, nearly all misprinted sheets are caught and destroyed before they leave the plant, and still more are caught during distribution or at the post office before being sold.
A much less common situation is for the invert to be embedded into the printing plate or stone, most famously the Inverted Swan of early Western Australia.
An invert may be characterized as an "inverted center" or "inverted frame" when the underlying paper is watermarked or otherwise carries a basic orientation. It is possible for a single-color stamp to be inverted relative to watermark, but this is called an "inverted watermark" rather than an "inverted stamp". Depending on the positioning of stamps within their sheet, the invert may be perfectly centered (as with the Inverted Jenny), or offset.
Not all inverts are spectacular. The Dag Hammarskjöld invert of 1962 consists only of a misprinted yellow layer, and it is not immediately clear that the white area is not a deliberate element of the design. Early Danish posthorn issues have an ornate frame that is almost perfectly symmetrical, and an inverted frame can only be detected by minute examination.
Overprints may also be inverted. Many of these are common, since the expedient nature of many overprints means that the production process is not so carefully controlled.
Rare inverts often have significant monetary worth. Inverted Jennies have long sold for over 100,000USD apiece, and the St. Lawrence Seaway inverts of Canada approach those numbers. High prices for inverts have tempted printing company employees to steal misprinted sheets from the printing plant and attempt to pass them off as genuine, as in the 1996 case of the "Nixon invert".