The term ‘fixed cost’ has been explained in the seminar as ‘a cost that remains

The term ‘fixed cost’ has been explained in the seminar as ‘a cost that remains unchanged when production activity changes over a period of time and over a certain relevant range’. He is unsure of what is meant by ‘period of time’ and ‘relevant range’ as he believes that there’s no such thing as a fixed cost and that all costs are variable in the long term.