Number of defined events occurring per __photon____absorbed__ by the system.

The integral quantum yield is
For a photochemical reaction,
The differential quantum yield is
where
is the rate of change of a measurable quantity (spectral or any other property), and
the amount of photons
(
or its equivalent
)
Notes:

- Strictly, the term quantum yield applies only for monochromatic excitation. Thus, for the differential quantum yield, the absorbed spectral photon flux density (number basis or amount basis) should be used in the denominator of the equation above when is either the number concentration (), or the amount concentration (), respectively.
- can be used for photophysical processes (such as, e.g., intersystem crossing, fluorescence and phosphorescence) or photochemical reactions.

Source:

PAC, 2007, *79*, 293*
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
* on page 406