The constant named after Edwin Hubble is the Hubble constant.
Usually written H₀, it describes the present-day relationship between a galaxy’s distance and its recession speed: v = H₀D. Its units are commonly kilometers per second per megaparsec, indicating how much faster a galaxy appears to recede for each additional megaparsec of distance.
The historical connection with the Doppler effect involves astronomical redshift. In 1912, Vesto Slipher measured Doppler shifts in the spectra of spiral nebulae and found that most were receding. Edwin Hubble later combined redshift-based velocity estimates with distance measurements, publishing evidence for the distance–recession relationship in 1929. Thus, Hubble is the namesake of the constant, although Slipher’s earlier observations were crucial.
The name can also cause a conceptual mix-up: the Hubble constant measures the current expansion rate, while the Hubble parameter changes with cosmic time. The value of H₀ remains an active research topic because different measurement methods produce slightly different results, a disagreement known as the Hubble tension.