Explain why some costs are considered to be variable and some fixed. How does time enter into the definition?

Some factors cannot be adjusted quickly in the short run. Costs associated with these factors are called fixed costs. Variable costs are costs incurred through the utilization of factors that can be readily varied in the short term (such as raw materials or labor). Variable costs are costs that increase as output increases, and decrease as output decreases. All costs are variable in the long run when firms have sufficient time to vary all factors of production.