A sequence of declarations, definitions, and statements enclosed within curly braces ({ }) is called a “block.” There are two types of blocks in C. The “compound statement,” a statement composed of one or more statements (see The Compound Statement in Chapter 5), is one type of block. The other, the “function definition,” consists of a compound statement (the body of the function) plus the function’s associated “header” (the function name, return type, and formal parameters). A block within other blocks is said to be “nested.”
Note that while all compound statements are enclosed within curly braces, not everything enclosed within curly braces constitutes a compound statement. For example, although the specifications of array, structure, or enumeration elements can appear within curly braces, they are not compound statements.