In C language, a union is a user-defined data type that allows the programmer to store different data types in the same memory location. Like a structure, a union can have multiple members of different data types, but only one member can be accessed at a time. This means that a union can be used to represent different data types at different times, depending on the application requirements.

The syntax for defining a union in C language is:-

union union_name

            {

                            data_type member1;

                            data_type member2;

                            …

                            data_type memberN;

            };

Here, union_name is the name of the union, and member1, member2, …, and memberN are the members of the union, each with a unique name and data type.

Structures and unions are powerful features of C language that allow programmers to organize and manipulate complex data in a simple and efficient manner. By using structures and unions, programmers can create custom data types that can be used to represent real-world objects or abstract concepts, making their programs more readable, maintainable, and efficient.