1. The #define command is a macro definition command in C language. It is used to define an identifier as a string. The identifier is called the macro name, and the defined string is called for replacement text. This command has two formats: one is a simple macro definition, and the other is a macro definition with parameters.

2. Define means that the value of a variable can be changed to the value we define, such as #definePI=14, then during the use of subsequent PI, the value of PI itself will always be 14. System variables are divided into four types: int integer variables. Variable for users to store integers.

3. There are three ways to use define: The first is to define the identifier. The valid scope of the identifier is the entire program. It looks like defineXXX and is often used in conjunction with #if.

4. Define is a preprocessing command in C language. It is used for macro definition, which can improve the readability of source code and provide convenience for programming. Preprocessing commands start with "#", such as the inclusion command #include, the macro definition command #define, etc. Generally placed in front of the source file, they are called preprocessing parts.