The rejection of all religious and moral principles, often in the belief that life is meaningless.
..and the more precise philosophical definition is given as:
The belief that nothing in the world has real existence.
And therein lies the problem, because in order to believe that something is meaningless, you first have to define ‘meaning.’ That’s when the ground gets shaky, as it always does when semantics come into play. It seems the best you can hope for is to conclude that whatever the nature of existence, understanding it will always be shaky.