A constant$-$cost industry is one in which:

a higher price per unit will not result in an increased output.

if $100$ units can be produced for $$100,$ then $150$ can be produced for $$150,200$ for $$200,$ and so forth.

the demand curve and therefore the unit price and quantity sold seldom change.

the total cost of producing $200$ or $300$ units is no greater than the cost of producing $100$ units.