An undergarment is a piece of clothing you wear next to your skin, under your other clothes — underpants, a bra, a vest, a slip. The meaning is simple; the interesting part is who says it. This is a formal, slightly old-fashioned word. Real people getting dressed say 'underwear' (which covers all of it at once) or the specific word — 'my socks', 'a bra'. 'Undergarment' shows up on clothing labels, in shop categories, in museum captions about historical dress, in laundry instructions, and in writing that wants to be polite about a slightly private subject. A doctor or an official form might use it to avoid sounding too casual. Because of that formality, it can sound funny if you use it in ordinary talk — like saying 'I must purchase an undergarment' when you mean 'I need to buy some underwear'. Note the countability difference, which is the real reason the word survives: 'underwear' is uncountable (you cannot say 'an underwear' or 'two underwears'), so when you need to count individual pieces, 'undergarment' or 'a piece of underwear' is what English gives you. In historical writing you meet it a lot, because past centuries had many layered undergarments with their own names — corsets, petticoats, chemises.

內衣（褲）——正式、書面的說法。平常講話台灣人說「內衣」「內褲」就好；undergarment 多半出現在服裝標籤、商品分類、或介紹古代服飾的文章裡，日常用會顯得文謅謅。