Difference between avoidable cost and unavidable cost?

Avoidable Cost = These are those costs which can be avoidable by doing or not doing any particular activity For Example :Direct Costs

Unavoidable Costs = These are those costs which are not avoidable whether do or donot initiate any activity For example: Fixed Cost

But sometimes fixed costs are also avoidable or unavoidable by doing or not doing any activity in these cases fixed costs are also avoidable costs.