(Argumentum) ad dictionarium

Latin: “(argument) by dictionary [definition]”. This involves looking up a term or concept in a dictionary, encyclopaedia or similar reference work in order to use the definition given there as an explanation.

This is not necessarily a bad thing; on the contrary, it is usually beneficial for a discussion if everyone first agrees on a common definition of the terms used. This is because some participants may initially have different ideas about what a term actually means. By referring to an authoritative source, such misunderstandings can often be quickly cleared up.

However, there are various situations in which the dictionary definition is not really helpful. These include, for example:

• The referenced definition reflects general usage. In the specific context, however, a more specialised definition (i.e. one specific to the applicable domain) would be more appropriate. Accident fallacy

• The dictionary contains an outdated definition, whereas a more recent meaning of the word would be relevant in the discussion. Etymological fallacy

• The dictionary definition is regarded as the sole authoritative source, and further discussion (e.g. as to whether the above-mentioned exceptions apply) is no longer allowed. Appeal to authority

Whether this approach is fallacious, and if so, which specific fallacy is committed, depends on the specific situation in question.