Which term means to serve as an omen or a warning of something?

The term "portend" is appropriate because it specifically refers to the act of serving as an omen or warning about a future event. In various contexts, when something is said to portend, it implies that it foreshadows or signifies a significant occurrence, often one that is concerning or negative. This word carries a connotation of forewarning, providing insight into what might come next based on current signs or events.

In contrast, the other terms do not align with this meaning. “Penitent” relates to feelings of regret for wrongdoing; “ponderous” describes something that is heavy or weighty, often in a physical or figurative sense; and “pliant” refers to something flexible or adaptable. None of these terms connote the idea of warning or omen that is encapsulated by the concept of “portend.” Thus, "portend" is the term that accurately captures the essence of serving as a predictor or herald of future happenings.