A condition in which output increases at the same rate as employment when the quantities of all inputs are increased or reduced at the same rate.

A condition in which, when a firm increases its plant size and labor employed by the same percentage, its output increases by the same percentage and its average total cost remains constant. (p. 278)

when all inputs are increased by a certain proportion, output increases by the same proportion

Constant returns to scale are technological conditions under which a given percentage increase in all of the firm's inputs (the long-run) results in the firm's output increasing by the same percentage. With constant returns to scale, the firm's long-run average cost stays the same as its output changes.