Real Dictionary

jacket
Potatoes baked in their jackets are baked with their skin on.
countable noun: A jacket is a short coat with long sleeves.
countable noun: The jacket of a book is the paper cover that protects the book.

jacket in American English
a short coat, usually with sleeves
to put a jacket, or coat, on
noun: a short coat, in any of various forms, usually opening down the front
transitive verb: to put a jacket on (someone or something)

jacket in British English
noun: a short coat, esp one that is hip-length and has a front opening and sleeves
verb: to put a jacket on (someone or something)

jacket in Chemical Engineering
noun: A jacket is a cover which is put around a vessel, usually to prevent loss of heat.

jacket in Mechanical Engineering
noun: A jacket is a cover or casing for the outside of something, for example the insulating cover of a boiler.