What is 18th letter of the greek alphabet?

The 18th letter of the Greek alphabet is rho (Ρ, ρ). It is pronounced like the English letter "r" but with a slightly rolled or trilled sound. Rho is one of the most commonly used letters in the Greek language and appears in many important words, such as "anthropos" (human), "Christos" (Christ), and "parousia" (second coming). Rho also has significance in mathematics, where it is used to represent the radius of a circle, as well as in physics, where it represents density. In ancient Greek culture, the letter was associated with the god Apollo and was used as a symbol for "rhombus," a shape with equal sides and opposite angles that add up to 360 degrees.