Thank you, I was just going to post this myself. To be clear:
Appeal to Definition
The dictionary definition of X does not mention Y. Therefore, Y must not be part of X.
Or more detailed:
Using a dictionary’s limited definition of a term as evidence that term cannot have another meaning, expanded meaning, or even conflicting meaning. This is a fallacy because dictionaries don’t reason; they simply are a reflection of an abbreviated version of the current accepted usage of a term, as determined through argumentation and eventual acceptance. In short, dictionaries tell you what a word meant, according to the authors, at the time of its writing, not what it meant before that time, after, or what it should mean.
Dictionary meanings are usually concise, and lack the depth found in an encyclopedia; therefore, terms found in dictionaries are often incomplete when it comes to helping people to gain a full understanding of the term. Or in this context, what the term means in the context of 5th edition DnD vs what it means outside of that context.
This is a very common fallacy, Arial Black demonstrated it earlier in the thread and continues to do so because he believes that the definition from the dictionary of the word instantaneous is more important and/or negates the definition of the term in the context of 5e. But you know, maybe once we get questions out over twitter we can resolve the discussion once and for all. Hint hint nudge nudge Arial Black.