Disagreement in opinion.
Example:
There was so much dissension at the meeting that nothing got done, and everyone went home angry.
Dissension is a common feature of our political system. One party suggests a new law or policy, and then the other party often dissents, arguing that the new law or policy will have a terrible effect on the country, and proposing a different new law or policy of its own. This leads the first party to dissent in turn, and so on. Things usually get worked out in the end. Since dissentious behavior of this kind keeps everyone on their toes, most people feel that it's a good thing overall. But not everyone agrees.