If you co-operate, you do what someone has asked or told you to do. He agreed to co-operate with the police investigation The plan failed because the soldiers refused to co-operate. + co-operation co-operation The police underlined the importance of the public's co-operation in the hunt for the bombers. = assistance
If you co-operate with someone, you work with them or help them for a particular purpose. You can also say that two people co-operate. The UN had been co-operating with the State Department on a plan to find countries willing to take the refugees The couple spoke about how they would co-operate in the raising of their child + co-operation co-operation A deal with Japan could indeed open the door to economic co-operation with East Asia