突然 (tū rán) means suddenly or all of a sudden. 突 suggests a sudden thrust or break through and 然 means in a certain state. Together they describe something happening abruptly without warning. It functions as both an adverb (突然发生, happen suddenly) and an adjective (突然的变化, a sudden change). Similar expressions include 忽然 (hū rán), which is slightly more literary, and 一下子 (yī xià zi), which is more colloquial.