The argument from a dictionary is a logical fallacy and happens when someone's argument is based, in a problematic way, on the definition of a particular term as it appears in a dictionary. The problem with these arguments:
<ul>
<li>Dictionaries are descriptive, meaning that they attempt to describe how people use the language. It is not prescriptive in that it instructs them how to do so in a definitive manner.</li>
<li>Dictionaries don't always reflect the meaning of words as they're used by people.</li>
<li>Different dictionaries can list different definitions for a given term, and may even have several definitions for the same word.</li>
</ul>

The argument from dictionary

Not every use of a definition is necessarily fallacious. If the definition is properly justified and is selected in a properly justified way, it is generally not fallacious. However, it is fallacious when at least one of the following conditions are true:
<ul>
<li>There is no valid reason for using the definition.</li>
<li>The definition was cherry-picked out of a range of possible definitions.</li>
</ul>

Appeal to definition: when the process becomes fallacious

When responding to appeals to definition, it is useful to know the terms denotation, which is the literal meaning of a word, and connotation, which is a feeling the word evokes beyond its denotation.
<ul>
<li>Explain why the use of the definition is inappropriate in this case.</li>
<li>Explain why the proposed definition is flawed. Other dictionaries may offer different meanings than the one your opponent has chosen.</li>
<li>You can sometimes benefit from using specific and relevant examples to show why such arguments are problematic.</li>
<li>You can ask the person using the fallacy to correctly justify their reasoning in light of your criticism.</li>
</ul>

Responding to an appeal to definition

<ul>
<li>Reportive definitions aim to accurately capture the meaning of a term as it's ordinarily used.</li>
<li>Precising definitions add relevant criteria to a reportive definition to make it more precise for a specific purpose.</li>
<li>Stipulative definitions are used to establish a specific purpose. For example, "for the purpose of the present document, the term 'contract' means..."</li>
<li>A persuasive definition is a stipulative definition that is dressed up as a reportive definition or as a claim in an argument. The terms are redefined to present one's preferred definitions as facts.</li>
<li>Ostensive definitions are based on examples of the word that is being described. For example, "liquid" could be "things like water and oil."</li>
<li>Misleading definitions relies on misleading language so that its intended meaning is different than the meaning that most people will use.</li>
<li>Operational definitions are used to define certain measures where an exact, reproducible definition is needed, such as in scientific studies.</li>
</ul>

Types of definitions

The structure and use of dictionaries have changed as new technologies developed.
<ul>
<li>Easily available online dictionaries make it easier for people to use and cherry-pick preferred definitions in their arguments.</li>
<li>Dictionaries can be quickly updated with new words or new meanings of existing words.</li>
<li>Dictionaries are able to list a wider range of meanings and connotations.</li>
</ul>

Modern dictionaries

"We should ignore the theory of evolution because the dictionary says that a theory is just an opinion that you have about something you can't prove."
The person using this fallacy is basing their statement on a specific definition of the word "theory" while ignoring alternative definitions that will better capture the meaning of the term as it's used in a scientific context.

An example of the appeal to definition

The Appeal to Definition Fallacy: When People Misuse the Dictionary

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