Stands for "Data Definition Language." A DDL is a language used to define data structures within a database. It is typically considered to be a subset of SQL, the Structured Query Language, but can also refer to languages that define other types of data.
A Data Definition Language has a pre-defined syntax for describing data. For example, to build a new table using SQL syntax, the CREATE command is used, followed by parameters for the table name and column definitions. The DDL can also define the name of each column and the associated data type. Once a table is created, it can be modified using the ALTER command. If the table is no longer needed, the DROP command will delete the table.
Since DDL is a subset of SQL, it does not include all the possible SQL commands. For example, commands such as SELECT and INSERT are considered part of the Data Manipulation Language (DML), while access commands such as CONNECT and EXECUTE are part of the Data Control Language (DML). Of course, if you are working with a database, knowing the names of each language is not important as long as you know the correct syntax.