Variable Costs
These are costs for a firm that are based entirely on activity by the firm. For example a variable cost for an air conditioner maker would how many hours of work they pay their workers. If they only want a few air conditioners produced they could pay very little, but if they wanted to have a large amount produced they'd have to pay more works, for more hours, and possibly overtime wages. This would raise the costs significantly, and would be considered as a variable cost to the firm.