To my mind, underwear is anything you wear under your clothes, right? Bras, panties, slips (does anyone wear slips any more?) camisoles, undershirts, girdles, underpants, maybe even socks.
Yet from context, some people seem to use the term underwear to refer exclusively to panties and underpants.
So, what’s underwear?
To me it’s just briefs or boxers for men and panties/bras/girdles for women.
Kiwi here. I’d say underwear refers to anything worn under clothes.
Undies, on the other hand, refers to men’s or women’s knickers/panties.
Underwear is…optional. 
OK, to me it’s underpants, and a bra if worn in conjunction with underpants.
Everything you said, except socks.
Brit here: same as MLS. Not socks.
Underwear are garments worn under clothing, often including those garments used to preserve modesty due to sheer clothing like chamisoles, or men’s undershirts, slips, half slips, girdles, garter belts etc. An exception occurs when referring to things meant to be shown off, like this, this, or these for example. Those are in a limbo, they aren’t meant to be worn alone, yet they are meant as show.
In the Wanna household, we are kind of loose with the term. Mr. Wanna is known to annouce that he’s out of clean “underwear,” and I know that he means briefs. I usually refer to his briefs (and my own bikinis or whatnot)as “underpants.” (I don’t use the term “panties” - it sounds like a “baby-word” to me, like “tummy” and “grown-up” - but I digress.) But sometimes I’ll say “underwear” instead, because we both understand the term that way. I think it’s because I never have occasion to wash any of his clothing in the laundry with my bras, slips, camisoles (I don’t wear girdles). On the other hand, there are some of his clothes I can include in a load with my underpants. Also, I was his undershirts (which he doesn’t wear that often) in the same load as his underpants, so just saying “do I need to wash your underwear?” does the job.
If I go to a store and ask were I can find the “underwear,” I know I can find underpants, bras, camisoles & slips in the same general region, so that does the job, too - no real reason to be specific in that case.
Underwear are the garments worn to protect the outer clothes from bodily discharges.
At least that’s the functional medical definition. Make of it what you will.
I’m thinking Victoria’s Secret has nothing to do with the medical definition.
I use it like YaWanna. Certainly if my husband asked me “What’s that on your underwear?” I wouldn’t look at my socks. In the general sense, it includes bras for me, but I also use it specifically instead of boxers or panties.
I would still say that VS is somehow relevant to “bodily discharges.”
I would never think of T-shirts or A-shirts as underwear because it’s quite common to wear them as outerwear.
Underwear is briefs.
on, my dear FCM, you have way too much time on your hands
like sandra_nz, I’ve always taken it to mean all undergarments, whereas “undies” specifically covers the “naughty bits”
So, a bib is underwear?
You do learn something new every day.
OK, let me re-phrase that. Underwear are the garments worn to protect the person’s outer clothes from their own bodily discharges.
There’s a freaking medical definition of underwear?
there isn’t in nursing, which surprises me.
Do you have a cite?
(not being a jerk, just amazed that there is a def.)
I agree with the definition. Sadly, too much underwear today is worn as outerwear. What is next? Polar fleece under your T-shirt?
As I said, it’s a functional definition. Sort of like age + BUN = lasix dose, or the only good admission is a dead admission. It’s Lore, not science. 
If I wear my panties on my head, then are they still underwear?
Are corsets and camisoles underwear?