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.