Plain English Meaning
When liquids like fuel, oil, or cleaning chemicals land on a composite part of the aircraft, they can soak in and weaken it. Some chemicals do real damage quickly; others cause slow problems if left alone. Spills need to be wiped up promptly and the area checked.
Full Definition
The contact of fuel, hydraulic fluid, oil, paint stripper, cleaning solvents, or other chemicals with composite aircraft surfaces, which can degrade the resin matrix and weaken the structure if not promptly cleaned and inspected. Some fluids — particularly paint strippers containing methylene chloride or methyl ethyl ketone — can cause severe damage to composite materials, while others may cause gradual softening or staining over time.