The word antique usually refers to an older object valued because of its aesthetic or historical significance. But to understand the more precise definition of the word used by dealers, you have to look at American history. Antique's definition changed in the 1930s. Then, as now, true antiques were considered artwork and came in duty-free. However, up until the 1930s the increasingly busy U.S. Customs Office had to consider what objects should be classified as authentic antiques.
At the time, the word had different meanings for different people. In European collecting circles, the word could describe an antiquity from ancient Rome or Greece. In the United States, with its much shorter view of history, the word antique could describe an object made as recently as the Civil War. Businessmen looking to skirt duties tried to use an even vaguer definition, using the word to describe any beautiful and valued item that was less-than-new.
Seeking clarity, and a guidepost for what to collect duty on, the Customs Office polled dealers for a definition and from these formulated one of its own. Antiques, they concluded, were objects that pre-dated the mass production of objects in the 1830s. Since the defining moment went back about 100 years, the office defined an antique as something made over 100 years ago. Duty was collected on objects younger than the century-old divider, and it still is.
The definition also did away with a lot of the arbitrariness that used to go into deciding what objects should be considered antiques. The definition eliminated the subjective judgments. By having a fixed time gauge in the definition, dealers and appraisers no longer had to judge objects by their artistic merit or their historical significance or how they were made. The definition has stuck and is now one that collectors and dealers use to separate an antique from a collectible.