A spot color is a specially mixed ink using in printing. Spot color inks come in a rainbow of colors, including some specialty inks such as metallic and fluorescent.
Unlike CMYK or process color which creates colors by laying down layers of just 4 specific inks, spot colors are pre-mixed and you use one ink for each color in the publication. You can also use tints of a spot color to get the appearance that you're using more colors without the expense of additional inks.
There are different brands of spot color inks. In the United States, the dominant spot color printing system is PANTONE. The Pantone Matching System or PMS consists of over 1,000 colors of ink. Other spot color systems include TOYO, DIC, and ANPA.