What term describes something that happens suddenly and unexpectedly?

The term that describes something that happens suddenly and unexpectedly is "abruptly." This word is specifically used to convey a sense of suddenness and a lack of warning, emphasizing that the event occurs quickly and without advance notice.

For example, if a situation changes abruptly, it can catch those involved off guard, highlighting the shock factor associated with it. In contrast, the other options describe different patterns or frequencies of occurrence. "Gradually" refers to something that develops slowly over time, which is the opposite of sudden. "Frequently" indicates something that happens often, while "regularly" suggests a predictable pattern or schedule, neither of which aligns with the idea of suddenness. Thus, "abruptly" clearly captures the essence of an unexpected occurrence.