In the case of General Motors, "Lambda" has no specific meaning beyond that it is the corporation's line of four crossover vehicles (Saturn Outlook, Buick Enclave, GMC Acadia and the Traverse, of course).
Lambda happened to be the next Greek-letter code name up for this new platform (There's already Delta, Theta, and a few others, I think).
And by "platform," it just means that all four of those CUV's are built around the same shared basic major components (engine, transmission, safety engineering, etc.) with different outward appearances.
duayned is correct. Nearly every car platform in GM now has a greek alphabet name. They switched to the greek alphabet (delta, epsilon, kappa, gamma, lambda, etc) once they went through the entire standard alphabet with their previous platforms, as you pointed out