Albedo is the fraction of incoming light a surface reflects, running from zero for a perfect absorber to one for a perfect reflector. Fresh snow sits near 0.8, ocean water near 0.06, and Earth as a whole averages roughly 0.3. The quantity matters far beyond bookkeeping: because ice reflects and open water absorbs, melting ice lowers albedo and accelerates further warming, one of the strongest feedback loops in the climate system. In planetary science, albedo helps infer what a distant surface is made of.