In the context of markets, equilibrium is when there's a balance between supply and demand, causing prices to stabilize. When there's an imbalance between supply and demand, prices tend to fluctuate ...
When the market price equates the quantity demanded to the total quantity supplied by the number of firms in the industry when each firm produces on its long-run supply curve.
The market price equates the quantity demanded to the total quantity supplied by the given number of firms in the industry, when each firm produces on its short-run supply curve.