What is 18th letter in the greek alphabet?

The 18th letter in the Greek alphabet is rho (Ρ, ρ). It is pronounced as "roh". Rho is often used as a symbol or abbreviation for density in physics and engineering, as well as for the correlation coefficient in statistics. In ancient Greek, rho represented the sound "r" and was used in words like "rhombus" and "rhapsody". In the Greek numeric system, rho has a value of 100.