plethysmography
n. the process of recording and measuring volume or volume changes in organs or body tissues, such as the blood supply flowing through an organ. The instrument used in this process is called a plethysmograph. Examples include the penile plethysmograph, which records changes in the size of the penis and thus measures blood flow and erection, and the vaginal plethysmograph, which records changes in the amount of blood in the walls of the vagina and thus measures sexual arousal.