Information quantity refers to the information measure or content needed to select an event from n equal possible events, that is, the minimum number of times to ask "yes or no" in the process of identifying a specific event among n events.
In information theory, the definition of mutual information is: I (x; Y) = h (x)-h (x | y), and the latter term on the right side of the number formula is called conditional entropy, which can represent a formula of discrete messages, and it represents the uncertainty of X after Y is known. Therefore, the mutual information I (x; Y) is the amount of information about the source X obtained when Y is received. Corresponding to mutual information, H(X) is often called self-information.