marginal

Marginal means so small as to hardly matter — or right at the edge of something.

The everyday sense is about size, so a marginal improvement is barely worth measuring. Politics has a precise and important sense, since a marginal seat is one that could change hands at the next election, and elections are fought almost entirely in them. Economics uses marginal cost for the cost of producing one more unit, which is one of the central ideas of the subject. Farming has marginal land, which is barely worth cultivating.