Schema can be called as collection of objects but it is interoperated at different diversity in major platforms such as below:
SQL Server: Schema is collection of objects referring to unique name within one database, one database can have multiple distinct schemas within and each schema can contain DB objects such as tables, view, procedures, functions etc.
Oracle: In Oracle, every user is referred to as schema and there can be multiple schemas under one DB and then same as in SQL Server, each schema can contain DB objects such as tables, view, procedures, functions etc.
MySQL: In MySQL schema and database are used interchangeably, schema is referred as database. You cannot introduce any schema within one database as in SQL Server or Oracle.
The summary is in databases schema is a collection of DB objects but it's behavior changes according to platform on which you are using it.