The word efface, in general english usage, means to cause something to fade or disappear 1-2.
In the context of radiology it should be taken to mean that a space or cavity has been obliterated by the external application of mass effect.
For example, a brain tumour, exerting mass effect on the adjacent brain, may efface the overlying cerebral sulci or adjacent cerebral ventricles. This implies that no CSF remains in the space. If there is incomplete obliteration of the space then one can use the term 'partial effacement'.
It is important to note that intrinsic obliteration, for example in the setting of congenital asymmetry of the ventricles, is not effacement.