Rho (uppercase Ρ, lowercase ρ or ; el, ρο or el, ρω, label=none) is the 17th 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
A glyph () is any kind of purposeful mark. In typography, a glyph is "the specific shape, design, or representation of a character". It is a particular graphical representation, in a particular typeface, of an element of written language. A g ...
, Ρ, as the distinct
Latin letter
P; the two letters have
different Unicode encodings.
Uses
Greek
Rho is classed as a
liquid consonant (together with
Lambda and sometimes the nasals
Mu and
Nu), which has important implications for
morphology
Morphology, from the Greek and meaning "study of shape", may refer to:
Disciplines
*Morphology (archaeology), study of the shapes or forms of artifacts
*Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...
. In both
Ancient
Ancient history is a time period from the beginning of writing and recorded human history to as far as late antiquity. The span of recorded history is roughly 5,000 years, beginning with the Sumerian cuneiform script. Ancient history cove ...
and
Modern Greek, it represents a
alveolar trill
The voiced alveolar trill is a type of consonantal sound used in some spoken languages. The symbol in the International Phonetic Alphabet that represents dental, alveolar, and postalveolar trills is , and the equivalent X-SAMPA symbol is r. I ...
,
alveolar tap Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* M ...
, or
alveolar approximant
The voiced alveolar approximant is a type of consonantal sound used in some spoken languages. The symbol in the International Phonetic Alphabet that represents the alveolar and postalveolar approximants is , a lowercase letter ''r'' rotated 1 ...
.
In
polytonic orthography, a rho at the beginning of a word is written with a
rough breathing
In the polytonic orthography of Ancient Greek, the rough breathing ( grc, δασὺ πνεῦμα, dasỳ pneûma or ''daseîa''; la, spīritus asper) character is a diacritical mark used to indicate the presence of an sound before a vowel, ...
, equivalent to ''h'' ( ''rh''), and a
double
A double is a look-alike or doppelgänger; one person or being that resembles another.
Double, The Double or Dubble may also refer to:
Film and television
* Double (filmmaking), someone who substitutes for the credited actor of a character
* ...
rho within a word is written with a
smooth breathing over the first rho and a rough breathing over the second ( ''rrh''). That apparently reflected an
aspirated or
voiceless
In linguistics, voicelessness is the property of sounds being pronounced without the larynx vibrating. Phonologically, it is a type of phonation, which contrasts with other states of the larynx, but some object that the word phonation implies v ...
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 (polytonic) or ρω/
ρο (monotonic).
Other alphabets
Letters that arose from rho include Roman
R and Cyrillic
Er (Р).
Mathematics and science
The characters ρ and are also conventionally used outside the Greek alphabetical context in
science and
mathematics.
*In the
physical sciences to represent:
**Densities:
mass density,
air density
The density of air or atmospheric density, denoted '' ρ'', is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variation in atmospheric pressure, temperature a ...
or
charge density
In electromagnetism, charge density is the amount of electric charge per unit length, surface area, or volume. Volume charge density (symbolized by the Greek letter ρ) is the quantity of charge per unit volume, measured in the SI system in co ...
(''ρ'')
**
Resistivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
(''ρ'')
**
Rho meson
Rho (uppercase Ρ, lowercase ρ or ; el, ρο or el, ρω, label=none) is the 17th 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 sa ...
(ρ
+, ρ
−, ρ
0)
**
General quantum states
**
Hammett Equation, ρ is used to represent the reaction constant, this is independent of the position and nature of the substituents of the benzene ring.
*In
mathematics to represent:
**A length coordinate in
polar,
cylindrical
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infini ...
,
spherical, and
toroidal coordinate systems, and
toroidal and poloidal coordinates of the Earth's magnetic field.
**The
correlation coefficient
A correlation coefficient is a numerical measure of some type of correlation, meaning a statistical relationship between two variables. The variables may be two columns of a given data set of observations, often called a sample, or two component ...
of a population parameter
**The
spectral radius of a matrix
denoted as
**The
plastic number
In mathematics, the plastic number (also known as the plastic constant, the plastic ratio, the minimal Pisot number, the platin number, Carl Ludwig Siegel, Siegel's number or, in French, ) is a mathematical constant which is the unique real so ...
**The
prime constant
The prime constant is the real number \rho whose nth binary digit is 1 if n is prime and 0 if n is composite or 1.
In other words, \rho is the number whose binary expansion corresponds to the indicator function of the set of prime numbers. Tha ...
**The
sensitivity to interest rates of a pricing function
**The expected return of a given policy (
) in
reinforcement learning, denoted
*In
economics to represent the discount rate of future pence cash flows
*In
molecular biology to represent the
Rho protein responsible for termination of RNA synthesis. In such occasions, it is often represented as , to avoid confusion with the Latin letter ''p''
*In
molecular biology to represent the
Rho family of GTPases, important for cytoskeletal dynamic regulation.
*In
ecology to represent the population damping ratio where ρ = λ1 / , λ2, .
*In
computer programming
**The lower-case rho "⍴" means reshape in the
APL programming language
APL (named after the book ''A Programming Language'') is a programming language developed in the 1960s by Kenneth E. Iverson. Its central datatype is the multidimensional array. It uses a large range of special graphic symbols to represent mo ...
, and by extension also queries shape
**The lower-case rho "ρ" is used for the unary
rename operation in
relational algebra
*In
statistics to represent
Spearman's rank correlation coefficient, commonly known as Spearman's rho
*In
options
Option or Options may refer to:
Computing
*Option key, a key on Apple computer keyboards
*Option type, a polymorphic data type in programming languages
*Command-line option, an optional parameter to a command
*OPTIONS, an HTTP request method
...
theory to represent the rate of change of a portfolio with respect to
interest rates
Chi Rho
The letter rho overlaid with
chi
Chi or CHI may refer to:
Greek
*Chi (letter), the Greek letter (uppercase Χ, lowercase χ);
Chinese
* ''Chi'' (length) (尺), a traditional unit of length, about ⅓ meter
* Chi (mythology) (螭), a dragon
*Chi (surname) (池, pinyin: ''chí' ...
forms the ''
Chi Rho
The Chi Rho (☧, English pronunciation ; also known as ''chrismon'') is one of the earliest forms of Christogram, formed by superimposing the first two (capital) letters—chi and rho (ΧΡ)—of the Greek word ( Christos) in such a way tha ...
'' symbol, used to represent
Jesus Christ.
Rho with stroke (ϼ)
The rho with a stroke through its tail is used for abbreviations involving rho, most notably in γϼ for γράμμα as a unit of measurement.
Character encodings
Greek rho[Unicode Code Charts]
Greek and Coptic (Range: 0370-03FF)
/ref>
Greek rho symbols
Coptic ro
Functional symbol
Mathematical rho
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.
See also
*
Р, р - Er (Cyrillic)
*
R, r - Latin
*
P, p - Latin
References
{{Wiktionary, Ρ, ρ
Greek letters