Rho

Rho (uppercase/lowercase Ρ ρ) is the 17th letter of the Greek alphabet. It is used to represent the "r" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 100. Letters that came from it include the Roman R and Cyrillic Р.
In statistics, the lowercase Rho ("ρ") is used to represent population correlation. In physics, the same letter is used to represent density.