What is a 'conscientious objector'?

The BBC would have you believe that a serving Royal Navy medic who is refusing to go to Afghanistan is a 'conscientious objector'. I would have thought that a 'conscientious objector' was someone who refuses to serve at all in the armed forces, on moral or religious grounds, when  conscription was in force. Leading Medical Assistant Michael Lyons joined the Royal Navy voluntarily so how can use this defence?

However the BBC are anti the Afghan campaign and so are sympathetic to any news that puts that war in a bad light.