Automation Testing means whatever the condition is executed by taking any help of automated testing software and any automation testing script then it means this is automation testing.
Suppose requirement to convert test cases into automated test script and now some test cases are written to test software and further test cases will be converted into automation test script then will be executed by integrating automation test framework then it means it is automation process to test application.
In simple way it is that process where test cases will be executed with using any automation software and whatever the test condition will be executed with integrating any automated testing framework and done by automatically then that is called automation testing.
With a simple practical example: Like a user registration form is implemented with some fields like “Name, Email and Phone” and test cases are created to execute. Suppose a positive test case is written like “Enter valid name in name field, enter valid email in email field and enter valid phone in phone field”, and when tester create automation test script and by running test script then during test execution values are inserted automatically and executed completely so it means test script is executed automatically and that is why it is called automation testing.