rho; Ρ, ρ +

(Greek: the seventeenth letter of the Greek alphabet; Ρ, ρ)

rho
1. The 17th letter of the Greek alphabet, represented in the English alphabet (P, ρ) as "r" aspirated.

Of Phoenician origin.

2. In the system of Greek numerals, it has a value of 100.
rho meson
A very short-lived unstable meson with mass 1490 times the mass of an electron.

It plays an essential role in the electrical structure of nucleons.

You can see all of the letters of the Greek Alphabet on this page.