Caption: "Uppercase and lowercase Greek letter omega, the 24th letter of the Greek alphabet, in Times New Roman font." (Slightly edited.)

The capital is Ω (see left on the figure) and the miniscule is ω (see right on the figure).

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.