nervous

This word primarily describes a state of apprehension or jitteriness. It captures the physical and mental tension that occurs when someone anticipates something stressful, such as an exam or a first date. While "anxious" often implies a deeper, more long-term dread, "nervous" is frequently used for immediate, situational stress.

In a biological context, it shifts from an emotion to a technical description of the network of nerves in the body. This usage is clinical and neutral, devoid of the emotional tension found in the first sense.

When applied to animals or movements, it describes a lack of stillness. It suggests a high-energy restlessness—like a bird about to fly away—where the subject is hyper-aware of their surroundings and prone to sudden, twitchy reactions.