The term “assault weapon” can only be defined in a political or legal sense. Outside the topic of gun control, there is no such thing as an assault weapon.
Even within that context, the term is extremely difficult to define. Realistically speaking, it is a completely subjective assessment of the appearance of a firearm, and it is applied to firearms that evoke impressions of modern military equipment, dangerousness, and maliciousness.
The difficulty in defining “assault weapon” is best exemplified by the failure of the U.S. Assault Weapons Ban of 1994, which attempted to define assault weapons based on a set of mainly cosmetic criteria. Manufacturers complied with the ban and continued to produce firearms that looked like assault weapons.
Perhaps the satirical definition promulgated by the pro-gun community is actually the most meaningful:
An assault weapon is an Evil Black Rifle.
Firearms that tend to be identified as assault weapons are functionally equivalent to other rifles, typically hunting rifles, that do not seem to be included within the scope of assault weapons. The difference seems only to be in appearance. Meanwhile, those same assault weapons do not function the same way as military rifles, which are invariably fully automatic. Fully automatic firearms are already restricted in the United States.
Creative writers have little use for the term “assault weapon” or “assault rifle” in description, the use of which would only signal the author’s political views or ignorance. However, the term can be useful in character dialog to indicate those things about the character.