On issues considered sensitive to the EU (including but not limited to: accepting new member states, foreign and security policy, police cooperation), a measure can only be passed if no state votes against it. This is known as "unanimity".
The alternative system of voting used in the EU is "qualified majority". For a decision to pass with a qualified majority, it has to have support from at least 55% of EU member states representing no less than 65% of the EU population.