What is the longest word and What is its definition?

Dichlorodiphenyltrichloroethane, one of the most well-known synthetic pesticides.
Other long Words include:

electroencephalographically
octamethylpyrophosphoramide
ethylenediaminetetraacetic
laryngotracheobronchitides
meningoencephalomyelitides
pneumoencephalographically
electrocorticographically
electroencephalographical
immunoelectrophoretically
panhysterosalpingectomies
syngenesiotransplantation
tetrabromophenolphthalein
ureteropyelonephrostomies
ventriculocisternostomies
cholangiopancreatography
cholecystoduodenostomies
electrocardiographically
electroencephalographies
hysterotrachelorrhaphies
laryngotracheobronchitis
lymphangioendotheliomata
mediastinopericarditides
pancreaticoenterostomies
pancreaticogastrostomies
pancreaticojejunostomies
pneumoencephalographical
polioencephalomyelitides
pseudohypoparathyroidism
thyroparathyroidectomies