The Greek letter **ρ** (rho) was derived from the ancient Phoenician letter 𐤓 (rēš), which is thought to have come from a pictogram of a head. As a numeral, ρ has a value of 100. Incidentally, this is our 100^{th} post.

The lower-case ρ (or its variant ϱ) has many uses in maths and sciences, such as:

- in electrical engineering and physics: resistivity (the reciprocal of conductivity
*σ*) - in electromagnetism: the volume charge density,
*ρ*_{q} - in molecular biology: the ρ factor, a protein involved in the termination of transcription
- in mathematics: the Dickman–de Bruijn function
*ρ*(*u*) - in mathematics: the plastic number, a constant with approximate value of 1.32472, the unique real solution of the cubic equation
*x*^{3}=*x*+ 1 - in mathematics: the prime constant, an irrational number with approximate value of 0.41468
- in mathematics:
*ρ*(*A*), the spectral radius of a matrix*A* - in organic chemistry:
*ρ*-value (the reaction constant) of the Hammett equation - in particle physics: rho mesons ρ
^{+}, ρ^{0}and ρ^{−} - in physics: mass density and air density
- in spectroscopy: rotational relaxation time
- in statistics: the Pearson correlation coefficient

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