Lumen, or "lm", is a unit of measure of light. It refers to the visible power output of a light. For example, it is common to list on the packaging of commercial light bulbs the amount of electrical power (watts) the light bulb requires and also the amount of visible light power (lumens) the light generates.
When working with security cameras, it is more common to refer to lux levels. A lux level refers to amount of light energy incident on a surface.
Formally, a lumen is the standard unit of luminous flux. See hyperphysics,phy-astr,gsu,edu/hbase/vision/lumpow,html for a formal physics definition.
