A Schema is a plan or scheme.

In relation to XML a Schema is a description of the content and format of a class of XML files. Unlike a DTD, a Schema is written in XML, so the data and the Schema for the data can be manipulated using the same ToolSet?. A Schema can also be significantly more flexible than a DTD.

In relation to SQL a Schema is a set of description of tables, described as a set of columns (with names and types) and constraints.