What it is: The refusal, for reasons of conscience, to fight and kill in war. The refusal is made by people who have been conscripted (called up for compulsory military service), or who are already in the armed forces, and think that killing is wrong.
What it means: COs may think that killing is wrong for moral reasons, and therefore that killing in war is just as immoral. Or they may have religious beliefs that say killing is wrong. Many countries now agree that it is a human right to be able to refuse to fight and kill in war. But there are still some countries where conscientious objection is punishable by law. Some of the irony of the COs' situation is shown in a cartoon of two men in a prison cell: one man tells the other, 'I'm inside for killing someone', and his cell-mate, a CO, replies, 'Funny, I'm here for refusing to kill'. Some COs have been granted exemption from serving in the armed forces. Some have been allowed to serve in the army as non-combatants. Others have been offered 'alternative service', compulsory labour (for example, farming, forestry, building, or road-laying) or other work that is useful to their country. Yet others have felt that alternative service is indirectly supporting the war, and have refused to do that too; these are the people who have been most likely to serve prison sentences.
Think about it: During the First World War very few British COs were given exemption from going to war. The rest had a very tough time - some even died as a result of harsh treatment in prison or work camps. They were also treated with contempt by soldiers and conscripts, and by the civilian population. It needed courage and determination to be a CO. During the Second World War there were hundreds more COs, and their treatment was not so harsh. Is standing by an unpopular principle still difficult these days? Or is it easier because being different is more acceptable? Does it make a difference whether the principle you stand by is a private matter or is one you want people to know about and share (like pacifism, for example)?