A measure of how much light is projected onto a surface. One lumen is the amount of light on a 1 foot square white surface located 1 foot away from a Standard Candle. A standard candle produces exactly 1 candle power of light. Note that candle power is the amount of light initially produced while lumen is the amount of light projected onto a surface. While at 1 foot these numbers are the same, the law of squares changes lumen output dramatically.
