An externality in business or economics is where an industrial activity has an unexpected side effect which does not figure in the cost of the goods and services involved. For example, I worked many years at a large mine. Just the existence of the mine there meant it was a no-hunting area so a side effect was that the moose used it as a refuge during hunting season which as a side effect was beneficial to the moose (and deer). Another example is that we used to crush mine rock for the haulroads for winter traction. As a result, it was found that the fines from this were concentrated with copper values so were put in the mill for processing-an unexpected outcome.