what is mean by gravitational waves?

what is mean by gravitational waves?
Gravitational waves are changes in the gravitational field, which propagate outwards much like the ripples on a still pond of water after the surface is disturbed. Gravitational waves carry energy and momentum and propagate at the speed of light, much like EM waves do. Mathematically, gravitational waves are timedependent solutions of the Einstein field equations, more precisely they are periodic oscillations of the curvature of spacetime.
However, thus far  as I understand it  we have failed to detect such waves.
We have thus far failed to directly detect them, yes. There is, however, plenty of indirect evidence for their existence  remember that gravitational waves carry energy away from their origin, so for example in the case of binary stars we can observe that effect in the form of a gradual decay of the stars' orbits. If you do the numbers, they exactly tally the predictions made by GR. Refer here :
PSR B1913+16  Wikipedia, the free encyclopedia
You're right that all earthbased experiments have so far failed to detect any gravitational waves. Further out, the HulseTaylor binary pulsar orbital decay measurements show agreement with GR's predictions to a fraction of a percent (work for which the duo won the 2010 Nobel in Physics). So at least there is that one good experimental dataset that indirectly shows the existence of gravitational waves.
Hello Raja, Gravitational waves the name implies are ripples in the curvature of space time that propagates as waves....and this was first predicted by Albert Einstein in 1916
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