Rho (; uppercase Ρ, lowercase ρ or ; Greek, Modern (1453-);: ρο or Greek, Modern (1453-);: ρω|label=none) is the seventeenth letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician letter res . Its uppercase form uses the same glyph, Ρ, as the distinct Latin letter P; the two letters have different Unicode encodings.
Rho is classed as a liquid consonant (together with Lambda and sometimes the nasals Mu and Nu), which has important implications for morphology. In both Ancient and Modern Greek, it represents an alveolar trill in Greek, Modern (1453-); pronounced as /r/, alveolar tap in Greek, Modern (1453-); pronounced as /ɾ/, or alveolar approximant in Greek, Modern (1453-); pronounced as /ɹ/.
In polytonic orthography, a rho at the beginning of a word is written with a rough breathing, equivalent to h (Greek, Ancient (to 1453);: ῥ rh), and a double rho within a word is written with a smooth breathing over the first rho and a rough breathing over the second (Greek, Ancient (to 1453);: ῤῥ rrh). That apparently reflected an aspirated or voiceless pronunciation in Ancient Greek, which led to the various Greek-derived English words starting with rh or containing rrh.
The name of the letter is written in Greek as Greek, Ancient (to 1453);: ῥῶ (polytonic) or ρω/ρο (monotonic).
Letters that arose from rho include Roman R and Cyrillic Er (Р).
The characters ρ and are also conventionally used outside the Greek alphabetical context in science and mathematics.
A
\rho(A)
\pi
\rho\pi
The letter rho overlaid with chi forms the Chi Rho symbol, used to represent Jesus Christ. It was first used by Emperor Constantine the Great. A can be seen on his standard known as the Labarum.
The rho with a stroke through its tail is used for abbreviations involving rho, most notably in Greek, Modern (1453-);: γϼ for Greek, Modern (1453-);: γράμμα as a unit of measurement.[1]
These characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.