Which word is used to describe something that signals an impending threat?

The word "ominous" is used to describe something that suggests the presence of danger or an impending threat. It conveys a sense of foreboding or warning, indicating that something bad may happen. This term is often utilized in contexts where there are signs or signals that create anxiety or apprehension about future events. For example, dark clouds gathering before a storm could be described as having an ominous quality.

In contrast, the other options describe feelings or states that are positive or neutral and do not convey any sense of impending danger. "Joyful" reflects happiness and positivity, "reassuring" conveys comfort and calmness, while "calm" denotes tranquility and peace. None of these terms evoke the sense of threat or foreboding that "ominous" does.