polymorphism: a single object can refer to sub-class or super-class, depending on the reference type. JDK:java development kit. JVM: java virtual machine JRE: JAVA runtime environment index: an index is a pointer to data in a table primary key: A primary key is a field in a table which uniquely identifies each row/record in a database table. Primary keys must contain unique values. It is used to recognize every column and row in the table. foreign key: is used to link different two tables.