Sentence:  We assume that average total unit cost in the relevant region of operation is constant with respect to quantity produced ( the average cost curve is horizontal , and therefore is identical with the <WSD>marginal</WSD> cost curve ) , and is the same for every firm ( and therefore for the industry ) . 
Meaning of the word marked with <WSD> tags:  the increase or decrease in costs as a result of one more or one less unit of output