sud·den 
/ˈsʌdn̩/ adjective
/ˈsʌdn̩/ adjective [more sudden; most sudden] : happening, coming, or done very quickly in a way that is usually not expected
▪ a sudden change/rise in temperature ▪ Sudden fame/success can be difficult to deal with. ▪ She had a sudden urge to be outside. ▪ His death was very sudden. ▪ a sudden turn in the road ▪ I was surprised by her sudden decision to quit. ▪ The director's sudden departure leaves the organization's future uncertain.
▪ a sudden change/rise in temperature ▪ Sudden fame/success can be difficult to deal with. ▪ She had a sudden urge to be outside. ▪ His death was very sudden. ▪ a sudden turn in the road ▪ I was surprised by her sudden decision to quit. ▪ The director's sudden departure leaves the organization's future uncertain.
all of a sudden
: suddenly
▪ I was walking down the street when, all of a sudden, it started raining. ▪ All of a sudden, I saw what he was looking at.
▪ I was walking down the street when, all of a sudden, it started raining. ▪ All of a sudden, I saw what he was looking at.
— sud·den·ness /ˈsʌdn̩nəs/ noun [noncount]
▪ the suddenness of the attack ▪ I was surprised by the suddenness of her decision.
/ˈsʌdn̩nəs/ noun [noncount] ▪ the suddenness of the attack ▪ I was surprised by the suddenness of her decision.
