The position taken in order to lie horizontally (an elaborate way to say "lying down"), often used for an x-ray. (ie. lateral decubitus film). This is also commonly used to define an ulcer (i.e., decubitus ulcer, also known colloquially as a bed sore or pressure ulcer) that forms from too much pressure on the skin as a result of lying down for extended periods of time (e.g., patients in a coma).