As per Glen Myers in his seminal work. The Art of Software testing.
Testing is the process of executing a program with the intent of finding errors.
As per William Hetzel
“Testing is any activity aimed at evaluating an attribute of a program or system. Testing is the measurement of software quality”
The IEEE defines testing as
“the process of analyzing a software item to detect the differences between existing and required conditions [that is, bugs] and to evaluate the features of the software item.”