Static Equilibrium

Equilibrium is the point reached when all forces acting upon an object equal zero in all directions.
For instance, if there is 20 newtons acting upon a box from the right and 20 newtons acting upon a box from the left, that box is said to be in static equilibrium. Another example of static equilibrium would be a free body diagram.

These objects are in equilibrium because the net force in each direction is zero. 

In other words, equilibrium is the point at which an object is acting with an acceleration of zero. 

Static equilibrium has an added criteria. The object or system must be at rest; the object must be static. There are many objects in a state of static equilibrium. For example, a stationary table. Because this table has a constant 0 velocity (and therefore a 0 acceleration), it is not crumpling in on itself due to gravity, nor is it being pushed on the left or right, it is in a state of static equilibrium. The force of gravity is acting down upon the table, but as Newton's law states, every action has an opposite and equal reaction, meaning that the earth is pushing back up, mitigating the force applied downwards by gravity. Bridges are another example of static equilibrium. The bridge itself is unmoving, and if the bridge does not fail, it is in a state of static equilibrium. When a car drives along a bridge, a new force has been introduced to the bridge. The bridge dissipates this new force, so that it is constantly in a state of static equilibrium. Suspension bridges, arch bridges, and truss bridges all redistribute forces differently, but they all still remain in a state of static equilibrium.  

Suspension Bridge 

Above is an example of how a suspension bridge distributes force so that it maintains static equilibrium. Force is distributed into the columns and outwards into the far suspension cables. The Earth pushes up against the force downwards in each column, resulting in a net force of 0. The suspension cables, to the far left and right, push the force into the short columns where the Earth again pushes up, mitigating the force downwards. Because this bridge has a net force of 0, and it is stationary, or static, it is in a state of static equilibrium.