For other uses, see Rho.

Rho (upper case Ρ, lower case ρ) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Semitic Rêš "head" (see Resh).

Rho is generally classed as a liquid (together with lambda and sometimes the nasals mu and nu), which has important implications for morphology. It is pronounced similarly to the letter r in languages with a Latin-derived alphabet. In polytonic orthography a rho at the beginning of a word is conventionally written with a rough breathing (equivalent to h) — — and a double rho within a word is written with a smooth breathing over the first rho and a rough breathing over the second — — apparently reflecting an aspirated or voiceless pronunciation in Ancient Greek, hence the various Greek-derived English words which start with rh or contain rrh. The name of the letter is written in Greek as (polytonic) or (monotonic).

Letters that arose from rho include Roman R and Cyrillic Er (Р).

Common notational uses

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lower-case rho ρ

The lower-case rho ρ has a special meaning in the APL programming language.

It is commonly used in physics to represent density(upper-case) and resistivity(lower-case), in mathematics to represent the radius in a system of spherical polar coordinates, and in economics to represent the discount rate of future cash flows. In such occasions, it is often represented as , to avoid confusion with the Latin letter p.

Greek letters

Ρ | Rho (lizherenn) | Ро | Ro | Rho (bogstav) | Rho | Ρω | Ρ | Rho | Rhô | Rho | Ρ | רו | Rho (letter) | Ρ | Rho | Ro (litera) | Ρ | Ro | Rhoo | Rho | Ρ