In a general sense digital refers to information or data that is stored or communicated as a sequence of discrete values, rather than some scale across a continual set of values (analog). A digital system may use any (or several) of many different numbering schemes, including decimal (base 10), octal (base 8), and hexadecimal (base 16), but for the most part we associate the binary (base 2) numbering system with digital as it is the most commonly used numbering system in digital hardware such as computers and other logic based systems.
For our purposes, digital refers to the representation of a varying physical property such as sound or light waves (as in digital audio or video), by means of a series of numerical values (in binary, ones and zeroes). These digits are grouped together in “words” to represent parts (intervals) of the complex character of the audio or video material.