is for X-ray
X-rays are a type of electromagnetic radiation. They can pass through all uncompressed gasses and some solid and liquid substances. They are commonly used to take images inside objects most notably for inspecting internal damage to the inside of the human body, usually damage to bones.
Some electrical components are known to give off X-rays and so it is important to take this into account during electronic manufacture processes. Small doses of radiation can be very damaging in the long term.
