The clothes you go hiking in are like portable shelter, your first line of defense against the elements -- not to mention your best chance at staying comfortable, no matter what nature throws at you. The very best layers are weatherproof, which means they're both waterproof and windproof.
(Sorry -- zombieproofing is not yet an option. Maybe one day!)
There's a pretty big difference between being proof against the elements or just resisting them, so if a garment is just water- and wind-resistant, I'd call it weather-resistant -- not weatherproof.
Ideally, high-end weatherproof garments will be breathable too. But weatherproofing and breathability, in particular waterproofing and breathability, are always a tradeoff. Today's breathable fabrics are getting more and more waterproof, but they're still permeable on some level.
The most common measure of a garment's weatherproofing is its waterproofness; here's how that's measured and what it means.
When Does Weatherproofing Count?
If there are two items of hiking gear you really invest in, they should be a solid pair of boots and a good weatherproof jacket. Said jacket will last for years and, if you hike for long enough, you'll eventually find yourself in a situation where you are grateful for its ability to keep you warm and dry, in spite of blowing wind, rain or snow.
(You can still save some money by checking the usual subjects for great deals.)
Any "outerwear" garment -- jacket, pants, hat and gloves -- can be weatherproof.