A conviction is something certain: a judgment of guilty in court and a strong belief are both convictions.
In the legal world, when a judge or jury convicts someone of a crime — finding them guilty — this is called a conviction. Prosecutors try to get convictions, and defense attorneys try to prevent them. Also, convictions are beliefs — principles. The United States was founded on many convictions, such as the belief in free speech and separation of church and state. When you have a conviction, you're certain of something.