Complex sentence: The gray (symbol: Gy) is the SI unit of absorbed radiation dose of ionizing radiation (for example, X-rays), and is defined the absorption of one joule of ionizing radiation by one kilogram of matter.
Split into simpler sentences: The gray (symbol: Gy) is the SI unit of absorbed radiation dose by matter due to ionizing radiation (for example, X-rays). One "gray" is the absorption of one joule of energy, in the form of ionizing radiation, by one kilogram of matter.