What is the definition or description of: mercury poisoning?

Mercury toxicity: It was popularized in alice in wonderland by the mad hatter. Mercury was used to help shape hats, i was told. It can be found in fish (tuna, halibut, king mackerel, tilefish, shark, swordfish & salmon), release from burning of fossil fuels especially coal, fluorescent light bulbs, some in dental fillings, old thermometers and blood pressure cuffs.