1. Not stable; lacking firmness or equilibrium; prone to change or collapse
The ladder is unstable and dangerous to use.
A escada está instável e é perigosa para usar.
2. Not constant or reliable; subject to sudden changes in mood, behavior, or circumstances
His emotional state has been unstable since the incident.
Seu estado emocional tem sido instável desde o incidente.
3. In chemistry and physics: lacking chemical or physical stability; easily decomposed or subject to change
The compound is highly unstable at room temperature.
O composto é altamente instável à temperatura ambiente.