(~s)
1.
A ~ is something which you see when it is extremely hot, for example in the desert, and which appears to be quite near but is actually a long way away or does not really exist.
It hovered before his eyes like the ~ of an oasis.
N-COUNT
2.
If you describe something as a ~, you mean that it is not real or true, although it may seem to be.
The girl was a ~, cast up by his troubled mind...
= illusion
N-COUNT: usu sing