Regression testing:
  The process of testing an application to identify an 
impact of the bug fixed poart of the existing build of the 
application on the other functionalities of the same build 
of application, or to test the impact of the newly added 
functionality (enhancements in the application)on the 
existing functionality of the current version of the 
application is Regression testing, (Impact analysis process 
is carried out in this case)

Retesting:
  The process of testing the application rigorously with 
all the possible inputs to gain more confidence on the 
functionality of the application.  This process is rarely 
carried out, where we have ample time.