1. not firmly fixed or balanced; likely to change or fail
The ladder was unstable and dangerous to climb.
A escada estava instável e perigosa para subir.
2. emotionally or psychologically unreliable; prone to sudden mood changes
His unstable behavior concerned his family.
Seu comportamento instável preocupava sua família.
3. in physics, not in a state of equilibrium; tending to change spontaneously
Unstable atoms emit radiation.
Átomos instáveis emitem radiação.