Testing the unchanged features to make sure that it is not broken because of the changes (changes means – addition, modification, deletion or defect fixing)

1st build – Customer gives requirements – development team start developing features – testing team start writing test cases – testing team write about 1000 test cases for the 1st release of the product and after execution of the test cases – the product is released – customer does acceptance testing – and the product is moved to production.

2nd build – now, customer asks for 2extra features to be added and gives the requirements for the extra features – development team start building the extra features – testing team start writing test cases for the extra features – about 200extra test cases are written – thus a total of 1200 test cases are written for both the releases – now testing team – start testing the new features using the 200 new test cases – once that’s done, then start testing the old features using the old 1000 test cases to check if adding new features has broken the old features. Testing old features is called regression testing. Once everything has been tested, now the product is given to the customer who does acceptance testing and then moves the product to production.

3rd build – after the 2nd release, the customer wants to remove one of the features (say Loans) – he removes all the Loans related test cases (about 100) – and then tests all the other features to check if all the other features are working fine. This is called regression testing.