In Oracle PL/SQL, a schema is considered to be the user account and all of the objects contained within it. That is, a schema is a collection of database objects owned by a given user.
The technical definition is "the set of metadata (data dictionary) used by the database, typically generated using DDL. A schema defines attributes of the database, such as tables, columns, and properties. A database schema is a description of the data in a database."
Note that technically, a schema doesn't have to contain tables (although the vast majority do).