marginal

Concise Oxford English Dictionary © 2008 Oxford University Press:
marginal/ˈmɑːʤɪnl/

▶adjective

• 1 relating to or situated at the margin: marginal notes.

  ■ relating to water adjacent to the land's edge or coast: marginal aquatics.

• 2 of secondary or minor importance.

  ■ (of costs or benefits) relating to or resulting from small or unit changes.

• 3 (of a decision or distinction) very narrow.

■ Brit. (of a parliamentary seat) having a small majority.

▶noun

• 1 Brit. a marginal parliamentary seat.

• 2 a plant that grows in water close to the edge of land.

– derivatives
marginality noun.