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An einstein is a unit used in irradiance and in photochemistry. One einstein is one mole of photons, regardless of their frequency. Therefore, one einstein equals Avogadro's number. Irradiance can be measured in einsteins per square metre per second.

It intervenes in photosynthesis, where the light requirement for the production of oxygen is a fixed number of einsteins (about nine einsteins per mole of oxygen formed), regardless of the frequency of the light.

It is named in honour of Albert Einstein, who explained the photoelectric effect and introduced the idea of light quanta, now called photons, in a 1905 paper.