Hexadecimal (or "hex" for short) refers to a number system with a base of 16 that is used in web design to represent colours. In contrast to the decimal number system, which has a base of 10 (0-9), the hexadecimal system uses a base of 16 and therefore uses letters from A-F in combination with the numbers 0-9.

In web design, colours are usually represented by a hexadecimal number consisting of six characters. Each character represents a part of the colour, starting with the red values, followed by green and blue values. Each colour part can have a value from 0-255, which is represented in hexadecimal numbers from 00 to FF. For example, the colour white is represented as #FFFFFF (all colour components are set to the maximum of 255), while the colour black is represented as #000000 (all colour components are set to the minimum of 0).

The hexadecimal system is an important concept in web design as it allows colours to be clearly defined and reproduced. By using hexadecimal values, web designers can ensure that colours are displayed consistently and accurately on all devices and browsers.