A lumen is a unit of measurement used to quantify the amount of visible light emitted by a source. It is part of the International System of Units (SI) and is used to express luminous flux, which is the total amount of light output from a light source as perceived by the human eye.
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The lumen is the International System of Units unit of luminous flux: . A source with constant luminous intensity within a cone of solid angle supplies into that cone. A source with the same luminous intensity in every direction supplies over a full sphere. [ref]
1 Watt equals 683 Lumens of light power at wavelength 555 nm. At other wavelengths 1 Watt is less Lumens as it takes into account the sensitivity of the average human eye.