What is the meaning of ρ?

density

Dickman function

(Can we verify this sense?)

Lower-case rho (ῥῶ), the 17th letter of the ancient Greek alphabet. It represented the alveolar trill /r/. It is preceded by π and followed by σ.

The lower case letter rho (ρο), the 17th letter of the modern Greek alphabet.

Source: wiktionary.org