Something or somebody that is marginal.

(politics) A constituency won with a small margin.

An adjective "marginal" refers to something that is located at or near a margin or edge, both in a literal and figurative sense, and can also mean sharing a border or being geographically adjacent.

An adjective "marginal" describes something that is determined by a small margin, close to being unacceptable or leading to exclusion, barely productive, or subject to a change in sitting member with only a small change in voting behavior.

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