Fixed costs

Costs that do not vary with output over the relevant range: rent, salaries of permanent staff, depreciation, insurance.

They are the source of operating leverage, and the reason two companies with the same revenue and the same gross margin can have completely different earnings volatility.

The distinction is a matter of time horizon rather than nature. Almost every cost is variable over five years, and almost none is variable over one month, which is why the useful question is what can be changed within the period being modelled.

Fixed in the short run, negotiable in the long one.