Unanimity means that all participants agree.
It is the decision-making method used for many sensitive issues in the EU treaties.
The future
The EU Constitution makes qualified majority voting the general rule for EU law-making.
For Treaty amendments, until now subject to a unanimous decision of the Intergovernmental Conference, Giscard d´Estaing and the EU Commission have proposed a super-qualified majority – this would completely remove the right to veto. However, this was not accepted by the Convention on the Future of Europe and has not been introduced into the EU Constitution. Instead, the Convention decided that the Prime Ministers can go from unanimity to qualified majority without new ratification. See Art. I-24.4 of the EU Constitution.
Links
See Voting in the Council and Treaty changes.