marginal

Written in the margin of a book.

(geography) Sharing a border; geographically adjacent.

Of a value, or having a characteristic that is of a value, that is close to being unacceptable or leading to exclusion from a group or category.

(of land) Barely productive.

(politics, chiefly UK, Australia, New Zealand, of a constituency) Subject to a change in sitting member with only a small change in voting behaviour, this usually being inferred from the small winning margin of the previous election.

(economics, not comparable) Pertaining to changes resulting from a unit increase in production or consumption of a good.