Which word describes a deep fear or anxiety?

The term "trepidation" effectively captures the concept of deep fear or anxiety. It originates from the Latin word "trepidare," which means to tremble or be alarmed. Trepidation conveys a sense of apprehension or nervousness about something that may happen, making it synonymous with anxiety or fear regarding future events or outcomes. This fits perfectly within the context of someone experiencing a profound sense of dread or unease.

In contrast, the other words do not possess the same connotation of fear. "Apathy" refers to a lack of interest or concern—not a feeling of anxiety—while "indifference" denotes a similar absence of emotional engagement. "Serenity," on the other hand, describes a state of calm and tranquility, which is the opposite of fear or anxiety. Thus, "trepidation" stands out as the word that encapsulates a deep-seated fear.