Anything sudden is abrupt, quick, or unexpected. A sudden rain shower during your picnic is disappointing, while a sudden drop-off in the road ahead can be dangerous if you're on your skateboard.
The stock market might take a sudden dive, panicking investors, or your favorite TV show could make a sudden change in plot, confusing you for a couple of episodes. Sudden has been used as an adjective since the 15th century, and it was once a noun, too, meaning "that which is sudden." Today the noun form is obsolete, except in the phrase "all of a sudden."