What keyword is used to define a data structure in ABAP?

In ABAP, the keyword used to define a data structure is "DATA." This keyword serves to declare variables and structured data types within the program. When you use the DATA keyword, you can specify the name of the variable, the type of data it will hold, and, if necessary, its initial value.

The DATA keyword is fundamental in defining both simple variables and complex data structures such as internal tables or user-defined types. For instance, you can define a structured data type using a combination of DATA and TYPE statements, where DATA is explicitly used for variable declarations.

In contrast, STRUCTURE refers more to the concept of a data structure rather than a keyword in ABAP. While you can create complex types using structure definitions, STRUCTURE itself is not a keyword used in creating a variable or data structure.

Additionally, DEFINE is not an ABAP keyword. It is typically associated with macro definitions in other programming languages, but in ABAP, you don't use DEFINE for creating data structures. TYPE, while relevant, is used more for defining types and not for the direct instantiation of variables.

Therefore, DATA is the correct choice when defining data structures in ABAP, enabling developers to create and manage various data types necessary for their applications.