testing the software/application repeatedly within a certain
period of time continuously with adding some extra
feature/new functionality  into it and compare it with the
existing one.

increamental testing is a way of integration testing in 
which first you test each module of the software 
indiviually then continue testing by adding another module 
to it then another.
Its proper definition is "A integration testing strategy in 
which you test subsystems in isolation, and then continue 
testing as you integrate more and more subsystems".

Advantages of increamental testing:-
when you find a problem, you can find the defect more 
easily because you have a better idea in which subsystem to 
look

there are two types of increamental testing:-

1.Vertical increamental testing
under this there are two testing strategies

i)Bottom-up testing
In this you start by testing the very lowest levels of the 
software using drivers, and then work upwards, as you 
integrate successive layers

ii)Top-down testing
you start by testing only the user interface, with the 
underlying functionality simulated by stubs, then you work 
downwards, integrating lower and lower layers