Omisus

Omisus is a term sometimes used in informal logic and argumentation to describe an argument that is flawed due to the omission of a crucial piece of information or a relevant consideration. The arguer presents a case that appears sound on the surface, but a vital piece of evidence or a key perspective is left out, significantly altering the conclusion's validity or persuasiveness.

The fallacy lies not in explicitly stating something false, but in creating a distorted or incomplete picture that leads the audience to draw an inaccurate inference. The omitted information is typically relevant and, if included, would weaken or even negate the argument.

This type of flaw can be intentional, used to manipulate or mislead, or unintentional, stemming from a genuine oversight or lack of awareness on the part of the arguer. Regardless of the intent, the resulting argument is weak and potentially misleading. It is related to, but distinct from, the fallacy of composition (assuming what's true of a part is true of the whole) and the fallacy of division (assuming what's true of the whole is true of a part). Omission, in this context, refers specifically to the absence of crucial evidence or perspectives.