You should consider a schema to be the user account and collection of all objects therein as a schema for all intents and purposes.
SCOTT is a schema that includes the EMP, DEPT and BONUS tables with various grants, and other stuff.
SYS is a schema that includes tons of tables, views, grants, etc etc etc.
SYSTEM is a schema.....
Technically -- A schema 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.