Stability
Définition : a) Partial constraints
A structure or one of its members may have fewer reactive forces than equations of equilibrium thar must be satisfied.
Exemple :

Définition : b) Improper constraints
Improper constraining by the supports causing instability although there may be as many unknown forces as there are equations of equilibrium.
This can occur if:
All the supports reactions are concurrent at a point.
The reactive forces are all parallel

Remarque :
If the structure in unstable, it does not matter if it is statically determinate or indeterminate. In all cases such types of structutres must be avoided in practice.