Page "Definition" Paragraph 36

# The definition must not be obscure.

The purpose of a definition is to explain the meaning of a term which may be obscure or difficult, by the use of terms that are commonly understood and whose meaning is clear.

The violation of this rule is known by the Latin term obscurum per obscurius.

However, sometimes scientific and philosophical terms are difficult to define without obscurity.

( See the definition of Free will in Wikipedia, for instance ).