As u/dolanbp mentioned, this is a 20mm round produced by Dura Co. Of Ohio. Per the markings on the projectile, it appears to be 20mm Ball. But I cannot guarantee it.
As someone who has experience with Unexploded Ordnance, I cannot stress enough that you do not want this round hanging around. While there is a primer strike, which indicates an attempt to fire, unless you definitely know it has been rendered inert there is no way to guarantee its safety. You can purchase inert rounds for $5-10 online, and not have to worry about it.
Looking at your post history, it appears you're Canadian (and possibly in Ontario). The Department of National Defense has been undergoing a massive project to identify and clear former wartime depots in the area. You should contact them to dispose of the round and, if you found it, report that location so they can properly clear the area and keep others from possible harm.
They can be contacted on the web or by phone at 1-800-207-0599.
The dimple in the center of the base means that the primer has been struck by a firing pin; in other words, the round has been 'fired', or the case has at the very least. There isn't really any way to tell if it's 'safe', however, as it could be an empty shell (no "gunpowder" in it), or it could be 'live' but was a misfire (didn't go off) and so retained the projectile at the top. My guess is though that if it's this old and hasn't gone off, it's safe, though I wouldn't blame you at all if you wanted to handle it cautiously and maybe get it inspected by a gun smith.
As to what it is, it's a 20mm cartridge. If it is indeed a US, WWII era shell, it could have been used in a P-38 Lightning (a fighter plane), but more likely was intended for use in the 20mm Oerlikon AA guns the US Navy mounted on pretty much every ship and boat it operated.
