Caption: "Uppercase and lowercase Greek letter lambda, the 11th letter of the Greek alphabet, in Times New Roman font."

The capital is Λ (see left in the image) and the miniscule is λ (see right in the image).

The capital letter Lambda Λ is the symbol used for the cosmological constant in cosmology.

Often the cosmological constant is just referred to as Lambda or Λ since that is easy to say.

The small letter lambda λ is the universal symbol for wavelength in physics.

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