What is the prefix for "two" or "double"?

The prefix that signifies "two" or "double" is indeed "bi-." This prefix is commonly used in various medical and scientific contexts to describe something that occurs in pairs or has a dual nature. For example, "bicycle" refers to a vehicle with two wheels, and "bifocal" denotes glasses with two different optical powers.

The other prefixes listed have distinct meanings: "mono-" means one, "penta-" refers to five, and "tri-" indicates three. Understanding these prefixes is essential in medical terminology as they help convey critical information about conditions, procedures, and anatomy.