Although tenuous doesn't have a fixed definition in science, it's often used, as OP said, to refer to the slightly elevated levels of free atoms and molecules around bodies like the Moon and Mercury. These "atmospheres" are thicker than open space but still about as good a vacuum as we can create in a lab, and essentially are vacuum for most aerodynamic purposes.
There are also bodies which are not enveloped in an atmosphere per se, but outgassing, volcanism, and electrostatically elevated matter periodically cause temporary pockets or plumes of "stuff" to be present near the surface. We've actually got planets with this level of "tenuous atmosphere" in the game (excluding the Europa-style plumes and the Moon -- the latter blocked because cities, not because atmosphere). You can see it in the faint haze and seasonal "crater fog" you get on some planets. We're still talking hundredths or thousandths of a bar at best. There's no need to model the effects of this on ships at the normal-space speeds we travel at near planets.
These may get upgraded, but, with the fact they already exist, I think the tenuous atmospheres of Odyssey have to then be those up to roughly modern-day Mars level. Which is about 1% Earth atmospheric pressure. Those without complex weather systems and large bodies of liquids. Maybe some light wind and dust plumes. Some wispy, scattered clouds perhaps.
I'd be disappointed if there wasn't some effect on the flight model at those pressures. Perhaps small, pseudo-aerodynamic craft like Eagles, Cobras and Vipers will have an advantage down there.
