Equilibrium can be spited in given types:

(i) Stable equilibrium: After showing a charged particle from it's equilibrium point, if it goes back then it is called in stable equilibrium. If U is the potential power then in case of stable equilibrium d²U/d²X   is +ve i.e., U is minimum.

(ii)Unstable equilibrium: After showing a charged particle from it's equilibrium point, if it never goes back then it is called in unstable equilibrium and in unstable equilibrium d²U/d²X    is -ve i.e., U is maximum.

(iii)Neutral equilibrium: After showing a charged particle fro it's equilibrium position if it neither comes back, nor moves away but remains in the position in which it was reserved it is said to be in neutral equilibrium and in neutral equilibrium d²U/d²X   is zero i.e., U is constant