Decubitus ulcer
A bed sore, a skin ulcer that comes from lying in one position too long so that the circulation in the skin is compromised by the pressure, particularly over a bony prominence such as the sacrum (sacral decubitus).
The Latin 'decubitus' (meaning lying down) is related to 'cubitum' (the elbow) reflecting the fact the Romans habitually rested on their elbows when they reclined.