A dissenter is someone who disagrees with commonly held opinions or beliefs. Except for one dissenter (your brother who dislikes amusement parks), everyone in your family is looking forward to going to Disneyland next summer.
If the U.S. Supreme Court makes a decision that's supported by eight of the Justices, the one Justice who disagrees with the ruling is a dissenter. This word is also used to mean "protester," or someone who stands up and takes action against a law or policy. In the 17th century, dissenter was used for someone who disagreed with a religious belief or doctrine.