The Greek letter Rho (ρ) is a letter that looks like a 'P' and is the 17th letter of the Greek alphabet. It is used in many different contexts, including mathematics, science, and engineering. In mathematics, it is often used to represent the radius of a circle or the rate of change of a function. In science, it is used to represent the density of a substance. In engineering, it is used to represent the resistance of an electrical circuit.