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 ר)
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 ρώ (monotonic).
Rho is used to designate a list of items in the APL programming language.
Letters that arose from rho include Roman R and Cyrillic Er.
Lower case ρ is used in physics to represent density and resistivity.
Rho - for other meanings.