What is the prefix that indicates two or a pair?

The prefix that indicates two or a pair is "bi-." This prefix is derived from Latin and is commonly used in various terms in both scientific and everyday language to signify something that is dual or formed of two parts. For instance, in medical terminology, "bicycle" relates to a vehicle with two wheels, and "biceps" refers to a muscle with two heads, illustrating the use of "bi-" to denote two.

The other prefixes convey different meanings: "mono-" signifies one, as in "monopoly," which means one entity has control. "uni-" also refers to one and is often used in terms like "unilateral," indicating something affecting one side. "tri-" denotes three, as seen in "tricycle," which has three wheels. Understanding these prefixes can help in deciphering words and their meanings across various fields, including sports medicine.