The longest word

pneumonoultramicroscopicsilicovolcanoconiosis (45 letters)
(/njuːˌmɒnoʊˌʌltrəmaɪkrɵˌskɒpɪkˌsɪlɪkoʊvɒlˌkeɪnɵkɒnaɪˈoʊsɪs/)
 (also spelled PNEUMONO­ULTRA­MICRO­SCOPIC­SILICO­VOLCANO­KONIOSIS)
a lung disease caused by breathing in particles of siliceous volcanic dust.
This is the longest word in any English dictionary. However, it was coined by Everett Smith, the President of The National Puzzlers' League, in 1935 purely for the purpose of inventing a new "longest word". The Oxford English Dictionary described the word as factitious. Nevertheless it also appears in the Webster's, Random House, and Chambers dictionaries.
超微粒硅酸盐尘埃沉着症, 简称火山肺矽病、火山矽肺病

• pneumono － 關於肺部的
• ultra － 超過
• microscopic － 極微小的
• silico － 硅（silicon），旧称矽。
• volcano － 火山
• coniosis － 塵埃引致的疾病

学术上更通用的名字叫做Pneumoconiosis（肺尘病）

HEPATICO­CHOLANGIO­CHOLECYST­ENTERO­STOMIES (34 letters)
a surgical creation of a connection between the gall bladder and a hepatic duct and between the intestine and the gall bladder.
This is the longest word in Gould's Medical Dictionary.

• 肝管胆管肠吻合术

Etymology:  From hepatos (liver) 肝+ ikos + chole (bile)胆汁 + angio (blood vessels)血管 + chole (bile) + kysta +enteros (intestine) 肠+ tomia (a cutting) 切