Equilibrium can be defined as a state of balance achieved by a structure or a state of rest occurred when all the forces are equal and opposite to each other.
What is Structural Equilibrium
In Structural analysis and structural design, we focus more on the magnitude, direction and the point of application of the forces as well as the resultant forces in order to ensure the state of equilibrium.

Fig.1: (a) The resultant force obtained after resolving must be provided with a reaction equal and opposite for maintaining the equilibrium (b) The moment at the cantilever support is the reaction formed that is equal and opposite to the force formed due to the point load at the end of the beam. 
In more easy words, a structural element will be in equilibrium when it react with an equal and opposite force to that applied on it.
Conditions of Equilibrium
For a rigid body to be in equilibrium, two conditions are:
 The vector sum of all the forces must be equal to zero. This will ensure translational equilibrium
 Next, the algebraic sum of all the moments of forces about any point must be equal to zero. This will ensure rotational equilibrium
Newton's Third Law of Action and Equilibrium
As per Newton's third law of motion, for every action, there is an equal and opposite reaction along the same line of action as the original force. Every element part of a structural system will have reactions that will contribute to the equilibrium of the whole system.
Also Read:
 1. Principle of Statics in Structural Analysis  Conditions of Static Equilibrium Sign Convention
 3. Introduction to structural Analysis
 4. Structural AnalysisMaterial Properties and Boundary Conditions
 5. Structural Analysis Idealizing a Structure for Analysis Part 3
 6. Simple Bending TheoryBending StressBending EquationDerivation
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