For women's clothing, a Medium typically corresponds to a US numeric size 8 to 10, which is roughly a 36-inch bust and 28–30 inch waist.
For men, Medium refers to chest measurement: 38–40 inches. Shirt-wise, this aligns with a 15–15.5 inch neck.
Letter sizing (XS-S-M-L-XL) is approximate by design. Brands group adjacent numeric sizes into one letter, which is why a Medium at one brand fits like a Small or Large at another.
If you're between sizes, your safest bet is to check the actual measurements (bust, waist, hip) on the brand's chart rather than rely on the letter.
Use SizeHub's converter to translate between letters, US numeric, EU, and UK across hundreds of brands.