What is knowledge?

Acquaintance knowledge

'knowing of', recognition, familiarity, direct experience. 

Ability knowledge

'knowing how', skills.

Propositional knowledge

'knowing that', knowledge of the truth or falsity of propositions. 

A necessary condition

Something which has to be true for something else to follow, but may not be enough on its own. E.g. being a man is a necessary condition of being a bachelor. 

A sufficient condition

A condition which once it is achieved, is enough for something else to follow. E.g. being an unmarried man is a sufficient condition of being a bachelor. 

Real essence

It can be given a real definition. There's a genuine difference between liquids on a molecular level that makes some liquids water and some not.

Nominal essence

No real definition possible. There is no genetic difference between weeds and non-weeds; the classification is culturally specific. 

Circularity

It must not contain the term being defined.

Obscure

The definition should not be more complicated or confusing than the original term, otherwise what's the point? 

Negative

You can't define a term by what it isn't. 

Ad Hoc

Your definition can't be tailored to counter specific problems, it should be a general one.