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:

• 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.
• Dictionaries don't always reflect the meaning of words as they're used by people.
• Different dictionaries can list different definitions for a given term, and may even have several definitions for the same word.

"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.

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:

• There is no valid reason for using the definition.
• The definition was cherry-picked out of a range of possible definitions.

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.

• Explain why the use of the definition is inappropriate in this case.
• Explain why the proposed definition is flawed. Other dictionaries may offer different meanings than the one your opponent has chosen.
• You can sometimes benefit from using specific and relevant examples to show why such arguments are problematic.
• You can ask the person using the fallacy to correctly justify their reasoning in light of your criticism.

The structure and use of dictionaries have changed as new technologies developed.

• Easily available online dictionaries make it easier for people to use and cherry-pick preferred definitions in their arguments.
• Dictionaries can be quickly updated with new words or new meanings of existing words.
• Dictionaries are able to list a wider range of meanings and connotations.

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