adjective. Happening without warning; unforeseen.

adjective. Happening or done without delay; hasty or immediate.

adjective. Characterized by sharp change in elevation; precipitous.

idiom. (all of a sudden) Very quickly and unexpectedly; suddenly.

Happening without notice, instantly and unexpectedly; immediate; instant.

Found or hit upon unexpectedly.

Hastily made, put in use, employed, prepared, etc.; quick; rapid.

Hasty; violent; rash; precipitate; passionate.

In zoology, abrupt; sharply defined from neighboring parts: as, a sudden antennal club; a sudden truncation.

noun. That which is sudden; a surprise; an unexpected occurrence.

Suddenly; unexpectedly.

noun. An unexpected occurrence; a surprise.

noun. sooner than was expected; without the usual preparation; suddenly.

noun. undefined

noun. undefined

adverb. Suddenly; unexpectedly.

adjective. Happening without previous notice or with very brief notice; coming unexpectedly, or without the common preparation; immediate; instant; speedy.

adjective. Hastly prepared or employed; quick; rapid.

adjective. Hasty; violent; rash; precipitate.

adjective. Happening quickly and with little or no warning, snell.

adverb. Suddenly.

noun. An unexpected occurrence; a surprise.

adjective. happening without warning or in a short space of time

"But the term sudden infant death syndrome, or SIDS, wasn't coined until the 1960s when it was declared a medical disorder."

gradual  unsudden  

abrupt  all of (on) a sudden  on such a sudden  upon all suddens  

abrupt  choppy  emergent  explosive  fulminant  

all of a sudden  sudden death  suddenly  suddenness  suddenwoven  

abrupt  intense  quick  slight  unexpected  

aback  abrupt  abrupt  abruptly  accelerated