In medicine, a catheter is a thin tube that serves a broad range of medical functions. They are inserted in the body to treat diseases or perform surgical procedures.

One popular type of catheter is the urinary catheter, which is inserted into the urethra in order to drain urine from the bladder, typically draining into a bag. This instrument is commonly used for people that are unable to empty their bladders, whether it is because of an infection, muscle tension or unconsciousness.