UDA is short for the Ulster Defence Association. It was the biggest of the Protestant paramilitary groups in Northern Ireland — 'paramilitary' meaning a group that arms and organises itself like an army, but is not the country's real army. It grew out of the conflict people call the Troubles, roughly the late 1960s to the late 1990s, in which one side wanted Northern Ireland to stay part of the United Kingdom and the other side wanted it joined to the Republic of Ireland. The UDA was on the stay-British side, and it fought republican groups like the IRA. It also killed many ordinary Catholic people who had nothing to do with any group at all, and that is a large part of why it was banned and treated as illegal. After the peace deal of 1998 it said it would stop using violence, and years later it did say it had put its weapons beyond use — though for a long time people doubted that, and some members moved into ordinary crime instead. You will meet this word almost only in news writing and history books about Northern Ireland, and the full name much more rarely than the three letters. British and Irish readers hear it as a heavy, political word; it is not a neutral label anyone would apply to themselves lightly.

北愛爾蘭「阿爾斯特防衛協會」（新教／親英陣營的準軍事組織，已遭查禁）。注意「準軍事組織」不是政府的正規軍，中文新聞有時也直接寫「民兵組織」。