Let $L$ be an almost calibrated exact Lagrangian brane representing a class in the (suitably enlarged) derived Fukaya category. Suppose for any almost calibrated exact Lagrangian objects $L_{1},L_{2}$ fitting into a distinguished triangle $L_{1}\to L\to L_{2}\to L_{1}[1]$, we always have

$$\hat{\theta}_{1}=\arg\int_{L_{1}}\Omega\leq\hat{\theta}_{2}=\int_{L_{2}}\Omega,\quad\text{resp. }\hat{\theta}_{1}=\arg\int_{L_{1}}\Omega<\hat{\theta}_{2}=\int_{L_{2}}\Omega,$$

then we say $L$ is Thomas-Yau semistable (resp. strictly stable). If $L$ fails to be Thomas-Yau semistable, we say it is Thomas-Yau unstable.