A person who campaigns for a person running for political office or works, or supports, in an organised and active way towards a goal; an activist.
Someone with experience in a certain field.
A person who has served in a military campaign.
A military veteran.
One who marches; one who participates in a march.
An inhabitant of a march (border country); specifically, a marcher lord.
A border territory, a march (now only in (attributive) use).