“White box testing” (also known as clear, glass box or structural testing) is a testing technique which evaluates the code and internal structure of the program.
In white-box testing the tester is concentrating on how the software does it. For example, a structural technique may be concerned with exercising loops in the software.
This method is named so because the software program, in the eyes of the tester, is like a white/ transparent box; inside which one clearly sees.
Definition by ISTQB
White Box Testing Techniques:
WHITE BOX TESTING ADVANTAGES
WHITE BOX TESTING DISADVANTAGES