What is an instance?
What is a SQL Server instance?
SQL Server expert Adam Machanic's definition of...(SearchSQLServer.com)
Installing two instances of SQL Server 2005 on same server
Can you install two instances of SQL Server 200...(SearchSQLServer.com)
An instance, in object-oriented programming (OOP), is a specific realization of any object. An object may be varied in a number of ways. Each realized variation of that object is an instance. The creation of a realized instance is called instantiation.
Each time a program runs, it is an instance of that program. In languages that create objects from classes, an object is an instantiation of a class. That is, it is a member of a given class that has specified values rather than variables. In a non-programming context, you could think of "dog" as a class and your particular dog as an instance of that class.