Many optic manufacturers are using the term waterproof to describe optics that are sealed from the elements and would normally keep water out. However, these optics are not actually waterproof, becasue water vapor still exists in the air contained inside the body. No big deal, right?
The only rifle scopes, binoculars, and other scopes that are completely waterproof are scopes that are also deemed as fogproof.
Picture yourself on a hot summer day with a glass of iced tea or lemonade. When the hot air comes in contact with the cold exterior of your glass, water condenses and makes the outside of your glass wet. The same thing can happen with sport optics.
In sport optics that are not waterproof and fogproof, air still exists inside the body between lenses, and in that air is water vapor. During rapid temperature changes (moving from a warm hunting cabin to cold outside air or vice-versa), the water vapor inside the optic will condense on the lenses, causing interior fogging, and may take hours, or days in some cases, to clear up. This can ruin a day of hunting or birdwatching.
Don't confuse this with condensation on the outside of the lenses, this is completely normal and there is no way to prevent this on any optic.
In waterproof and fogproof optics, the air inside the body has been replaced with an inert gas, usually argon or nitrogen, and purged of water vapor. Without water vapor, the inside of the lens can never fog.
So if you're looking for a waterproof rifle scope or waterproof binocular, make sure you get one that is also fogproof.