The definist fallacy occurs when an arguer asserts that because something cannot be defined with precision, it therefore does not exist, or that because a person cannot provide a proper definition of a term, that therefore the person cannot claim to know what the term corresponds to in the world.

Here is an example:

Zelda: What qualifies as good music?

Bern: I can't give you characteristics, but I know it when I hear it.

Zelda: Oh, so, you really don't know.

Zelda is presuming that Bern's inability to articulate what counts as "good music" means he simply doesn't know what it is. This confuses Bern's conceptual analysis (and linguistic) skills with his capacity to recognize something with which he is familiar.

Here is another example:

Theologians have never settled on a definition of "God" which spells out all of God's essential properties in a way that is clear and comprehensible. ​Therefore, the word God is meaningless, and so is the statement "God exists."

Similar difficulty in definition may be found with the concepts of a photon, gravity, space, time, emotion, will and the psyche (or self). The failure of experts to settle on a clear and complete definition of such things does not by itself indicate that the object in question does not exist, or that all mention of it is meaningless; it may instead indicate limitations of human concepts to grasp entities that are more subtle in nature than common physical objects.

Note that the term "definist fallacy" is also sometimes used to refer to persuasive definition.