@RedDarkSwordsman: I'd like to morph your proposed description into "A soft, cylindrical sex toy you put the dick in to stroke it with."

You need a better description because dildos and sometimes vibrators would technically count under this. Plus you are ignoring that onaholes are also used in futa porn. I would suggest something like. A cylindrical sex toy that is modified after the female genitals or in certain cases, the anus. Used to stimulate one's penis (again, to include futa)

When I think of an onahole, I think of a pink, tube-like, slightly see-through masturbator with a hole through it and a jello-ish wobble to it; or a variation of that. There are many other sextoys for men, like fleshlights, prostate stimulators, sounding rods, sex dolls, tenga eggs, cockrings, breeding mounts... not to mention all the buttplugs and anal toys. But I think I am the minority.

to be fair it's a bit vague, it's not the only toy for men