Greek letter ga

    Caption: "The capital lower-case "Greek letter omega, the 24th and last letter of the Greek alphabet, in Times New Roman font." (Slightly edited.)

    The capital is Ω (see left in the image) and the lower-case is ω (see right on the image).

    The capital Ω is the symbol used for the density parameter in cosmology.

    Often density parameter is just referred to as Omega or Ω since that is easy to say.

    Credit/Permission: Derrick Coetzee (AKA User:Dcoetzee), User:F l a n k e r, 2006 / Public domain.
    Image link: Wikipedia: File:Omega_uc_lc.svg.
    File: Hellas file: greek_letter_lambda.html.