For women, 1X is roughly equivalent to sizes 14 to 16, defined as 41.5 to 45 inches bust, 33.5 to 37 inches waist, and 44 to 47.5 inches hips. Size 1X is usually considered a plus size and can be slightly larger than XL, although sizes vary by brand.
As there is no standard definition of sizes, apparel companies define sizing at their own accord. Because of this lack of standardized definitions, a person wearing a size M in one brand may need to wear an L or XL in another. Given this large variance, it's important to try on clothes before buying them.