Illuminance refers to the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface. Illuminance also correlates with how humans perceive the brightness of an illuminated area when wavelength-weighted by the luminosity function. The unit of illuminance is Lux.
Many people usually confused this term with Luminous Flux, Luminance and Luminous Intensity(lm). From the above figure we can see, Luminous Flux refer to the total light emitted by a light source in all directions; Luminous Intensity refers to the light emitted towards a specific direction with a given angle, while the Luminance refers to the light reflected to human eyes.