The cycle of chemical reactions during photosynthesis which releases oxygen and reduces carbon dioxide to glucose (carbon fixation). Named after its principal discoverer, Melvin Calvin.

The cycle works like this:

CO2 -> ribulose bisphosphate -> 3-phosphoroglycic acid (PGA) -> 1,3-bisphosphoroglycic acid (DPGA) -> 3-phosphoroglyceraldehyde (PGAL) -> fructose, glucose, and ribulose phosphate -> ribulose bisphosphate, which is now ready to take on some more CO2.

Source: John W. Kimball's Biology Pages, which note that "All the reactions of carbon fixation occur in the stroma of the chloroplast".

This cycle is also known as light independent, non-light requiring, or dark reactions, since the energy is provided by ATP or NADPH molecules which are created by the light reactions. "Thus the Calvin cycle is inseparable from the light reactions...they only occur in the light! This is why "dark reactions" is a poor name for the Calvin cycle", (L. Taiz and E. Zeiger, Plant Physiology, 2nd ed., 1998)

Melvin Calvin and Andrew Benson worked this out by studying the green algae Chlorella at UCB.