A "refractor" is a baseball card manufactured by the Topps trading card company. It is a hard-to-find special card randomly inserted into packs of Topps's various trading card products.
Refractors were first introduced into packs of Topps Finest baseball cards in 1993. Since then, the company has included them into its other Finest brand of sports products such as basketball and football, and in some of its nonsports products as well.
A refractor is known for its colorful shine. The cards reflect light, often in a rainbow of colors, making them easy to differentiate from regular Topps Finest cards. The cards usually say "refractor" on the back as well.
These cards are generally scarce, making them much more valuable than the standard card of each player in the sets. Topps usually lists the odds of finding a refractor in each of its Finest products.
Refractors are the same size as standard trading cards -- about 2.5" by 3.5".
The initial set of Topps Finest baseball was not initially well received until after collectors discovered how scarce they were and how hard to find the refractors were. Refractors helped usher in the insert card craze that now predominates the market.