A cat does not have one universal “tail dictionary.” The useful unit is the whole body: ears, pupils, whiskers, tail, muscle tension, posture and context. One signal can mean several things; several signals pointing the same way are much more informative.
Four channels to watch together
Ears
Upright, forward ears often accompany a relaxed or attentive state. Flattened or sideways ears can signal fear, stress or defensive aggression.
Eyes
Soft eyes can accompany relaxation. Dilated pupils may occur with fear or arousal—but also in low light or excitement, so context matters.
Tail
A relaxed tail differs from a tucked tail, a puffed tail or rapid lashing. Tail movement should be interpreted with the rest of the posture.
Body posture
A crouched cat trying to look small is communicating something very different from a stiff, forward-moving cat trying to look larger.
Relaxed, fearful and aggressive can overlap—but not completely
Relaxed
Normal pupils, upright ears, loose muscles and an unguarded posture are common signs.
Fearful
Crouching, tucked tail, flattened ears and wide pupils often appear together.
Aggressive or highly defensive
Hissing, growling, swatting, arched posture, piloerection and tense movement are warnings to stop approaching.