A unionist is a person who believes in trade unions — someone who thinks workers should organise together, and who supports union action such as bargaining and striking. It is also used as an adjective, as in "unionist leaders" or "a unionist tradition". Often it points to someone active, not just sympathetic: a union member who does the work of the union. In older writing you meet "trade unionist" a great deal, and that longer form is clearer and still common in British English. As with unionism, the word can lean warm or cold depending on the speaker. In a friendly mouth it means a loyal, brave worker; in a hostile paper it can be a quiet way of saying "troublemaker". Be careful of one thing: with a capital U, a Unionist in Northern Ireland is not a union member at all, but someone who wants Northern Ireland to stay in the UK — and there the word is also part of party names, the Ulster Unionist Party and the Democratic Unionist Party. In American history, a Unionist was someone who supported the North, keeping the United States together during the Civil War. Same spelling, three quite different people, so let the surrounding words tell you which.