Humidity usually refers to the how close the water vapour in the air is to condensing out. In everyday use somebody might describe the weather as humid: that's the sticky sort of weather, when your sweat can't evaporate because the atmosphere is already saturated; weather that makes wood swell and doors jam. People wouldn't normally speak of the weather not being humid, but they might speak of "dry heat", which is generally more tolerable. Note that this everyday use only really refers to hot weather. A meteorologist might talk about humidity at any temperature, but most folks won't. Hydration is even more technical, and usually refers to the amount of water in a solid. The only place I've come across it outside Chemistry lessons is in the gym, where it's about drinking lots of water ("Got to stay hydrated!"). Its opposite, dehydrated, is used more: of food, where one is expected to add water to rehydrate it, and of people dying of thirst. You might also come across "dessicated", of food that you probably won't rehydrate ("dessicated coconut"). If dessicated is used of a person, they're probably long dead and mummified -- it's that dry.
Humidity refers to the water content of a sample of atmosphere. Hydration refers to water bound to materials, usually solids. In chemical terminology, hydration typically refers to an exothermic reaction, often reversible, between specific chemical compounds and water.
i have to wonder if you'd looked them up in a dictionary before posting this question, yokone... it would've been a lot quicker than waiting 3-4 hours to get the same definitions here...
In healthcare and first aid, we use the term hydration to refer to keeping an adequate amount of liquid in the body for survival. I always think of humidity as how much moisture is in the air.
I agree Unless after looking it up you didn't understand the difference or some more specific question. Define______ in search engine usually will give a quick set of definitions from various sites, you don't even have to click on it and you don't even have to find and open those old card board covered paper things that sit on a shelf, oh yea, books or a dictionary. I always read a few definitions just to be sure one site doesn't have a mistake or miss a definition that might be important.
