Which term refers to a signal of something significant or ominous that is to come?

The term "portent" specifically refers to an indication or omen of something significant or ominous that is about to happen. It is often used in contexts where an event or sign suggests the coming of a future occurrence that has considerable importance or weight, typically something foreboding. This term embodies the essence of foretelling or signaling impending events, particularly those with a serious or grave nature.

In contrast, other options do not share this particular connotation. A plea generally implies a request or appeal, often associated with desperation or urgency, rather than an ominous signal. Pivotal suggests something of crucial importance or significance but lacks the foretelling aspect associated with a portent. Plummet refers to a rapid fall or decline, which describes an action rather than a signal or indication of future events. Thus, "portent" is the most fitting choice for conveying the idea of a significant or ominous signal regarding upcoming developments.