A list of all characteristics which are each necessary and collectively sufficient to identify all of the things to which the defined word apply, while not including any things which do not. Many people use the phrase "necessary and sufficient" as a benchmark for remembering what an essential definition is. For example, the essential definition of "cell" (as used in biology) might be something like this (which I've borrowed and modified from Merriam-Webster Online):

    1. A mass of protoplasm
    2. Bounded externally by a semipermeable membrane
    3. Which is the smallest structural unit of living matter capable of functioning independently.

This definition is an "essential definition" because it does not allow for things that are not cells (like mitochondria) to mistakenly be included in the group "cells", because they are not independently functioning. At the same time it does include every example of a "cell" as the concept is understood.

Though this term, "essential definition" is used chiefly in philosophy, a definition which is not an 'essential definition' is, in my opinion, not really a very good definition at all. Since my endeavor in this node is to define the term "essential definition", and given how interesting self-reference is, I attempt below the essential definition of "essential definition":

essential definition

A collection of criteria which identify the meaning of a given term such that:

    1. Each criterion must be met in order for a thing to qualify as a legitimate example, AND
    2. There exist no other criteria which must also be met in order for all legitimate examples to be included.

Now, let's look at an example of a poorly-written essential definition. Let's define the word "pencil".

*pencil: a writing instrument which,

    1. Can generate marks which are eraseable
    2. Has the same general shape as a pen
    3. Is of correct size to be wielded by a human being

Now, the definition above may seem OK, but it fails to exclude some non-pencils: For example, a pen using eraseable ink would qualify as a "pencil" using this definition, and so would whiteboard markers. And even chalk. The definition needs some fixing -- it needs more criteria, so that we exclude things like whiteboard markers, chalk, and eraseable pens. Let's try again:

*pencil: a writing instrument which,

    1. Can generate marks which are eraseable
    2. Has the same general shape as a pen
    4. Can be sharpened using a grinding device or a knife.

Better, right? Nope! Now we've gone too far in the other direction. We probably don't have any non-pencil items in this definition, but the problem is now that we've excluded some things which ARE pencils. For example, mechanical pencils are still pencils, but one can't sharpen them, so they'd be excluded. And furthermore, I can conceive of a pencil which might use something other than lead or graphite to make marks; charcoal pencils, or maybe even a pencil that makes its marks with some other substance.

Generating an essential definition is often tricky, but when it's absolutely impossible to create one, there's a good chance that the thing you're trying to define doesn't really have a bounded, discrete, objective existence at all. In the case of pencils, what it means is NOT that there is no such thing as a pencil, but that the concept of what IS a pencil and what IS NOT a pencil is a matter of social consensus or of mental construction -- not any objective reality.

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