An X mark (also known as a cross, cross-mark, x, ex , X, ✕, ☓, ✖, ✗, ✘, etc.) is used to indicate the concept of negation (for example "no, this has not been verified" or "no, I don't agree") as well as an indicator (for example in election ballot papers or in x marks the spot). Its opposite is often considered to be the check mark or tick (or the O mark used in Japan, Korea and China). In Japanese, the X mark (❌) is called "batsu" (ばつ) and can be expressed by someone by crossing their arms.
It is also used as a replacement for a signature for a person who is blind or illiterate and thus cannot write his or her name. Typically, the writing of an X used for this purpose must be witnessed to be valid.
As a verb, to ex (or x) off/out or to cross off/out means to add such a mark. It is quite common, especially on printed forms and document, for there to be squares in which to place x marks, or interchangeably checks.
It is also traditionally used on maps to indicate locations, most famously on treasure maps and as a set of three, and to mark jugs of moonshine for having completed all distillation steps, while also signifying its potency (as high as 150 proof) relative to legal spirits, which rarely exceed 80 proof (40% ABV).
The mark is generally rendered with a less symmetrical form than the following cross-shaped mathematical symbols: