A built-in function is an expression in which an SQL keyword or special operator executes some operation.
Built-ins are SQL92Identifiers and are case-insensitive. Built-in functions use keywords or special built-in operators and those function can be used within the programming environment provided by the Oracle/PLSQL database, such as stored procedures, functions, triggers, etc.
For easy reference, Here all built-in-functions are divided into several types based on categories such as string/character, conversion, advanced, numeric/mathematical, and date/time.