Grays (Gy) are a unit of measurement used to quantify the amount of ionizing radiation absorbed by an object or person. One gray is defined as the absorption of one joule of radiation energy per kilogram of matter. This measurement is crucial in fields like radiation therapy, where precise dosages are necessary for effective treatment. The term "gray" is named after the British physicist Louis Harold Gray, who made significant contributions to the study of radiation. Understanding grays helps medical professionals assess radiation exposure and its potential effects on human health, ensuring safety in various applications, including radiology and nuclear medicine.