happening unexpectedly
"suddenly she felt a sharp pain in her side"
on impulse; without premeditation
"he decided to go to Chicago on the spur of the moment"
"he made up his mind suddenly"
Happening quickly and with little or no warning; in a sudden manner.
"The lights flickered out suddenly, leaving us in total darkness without any warning."
In plain English: Suddenly means something happens very quickly and without any warning.
"The phone suddenly rang while I was taking a shower."
Usage: Use suddenly to describe an event that occurs unexpectedly without prior notice, such as the lights flickering out mid-sentence. It functions primarily as an adverb modifying verbs rather than nouns or entire clauses.
The word suddenly comes from the Middle English sodenly, which was formed by adding the suffix -ly to the root for "sudden." It entered modern English with essentially the same meaning it held centuries ago when describing something that happens quickly or unexpectedly.