I have searched about automation testing and got idea what it is! but have not found any proper definition regarding AUTOMATION TESTING. Please help me
I assume you meant "Automated Testing" rather than "Automation Testing". Here's the definition I use in my daily activities:
Testing which is performed, to a greater or lesser extent, by a computer, rather than manually.
It's part of a Glossary of Terms I have developed over the years that comes in handy when answering this sort of question. http://www.allthingsquality.com/p/testing-terms-glossary.html
I use a Glossary internally within my QA Team, and externally as well. It often helps when everyone on your team uses the same words for the same terms.