Lumen maintenance is the amount of light output of a light source when it is brand new, compared to the amount of light output of the same source at some time in the future.
E.g. if a luminaire's lumen output is 1000 lumens when it is brand new, and produces 800 lumens after 20,000 hours, its lumen maintenance value is 80% at 20,000 hours. According to this, there are different types of light sources, according to lumen maintenance: the most often used are L70, L80 and L90.
Figure 1. is an example for lumen maintenance diagram for an LED Infusion M4500 module. It shows how the module performs at an LM-80 testing. (LM-80 is a process approved by IESNA – Illuminating Engineering Society. It measures the lumen depreciation pf LED light sources and helps in rating its estimated lifetime.)
This product has an L85-rated lumen maintenance value – in this case, it means that after 50,000 hours of use, the luminaire is still going to have 85% of its lumen output.