A security mechanism gives a state the right to re-negotiate or opt-out of a treaty (derogate) in certain unforeseen situations.
The EU agreements with the former applicant countries (now members of the EU) contain a special security mechanism. It will be possible to suspend a part of the agreement if the applicant countries do not fulfil the agreed acquis.
There is a broad security mechanism for the single market and another for Justice and Home Affairs. The special security mechanisms are "one-way streets", which can only be called upon by the at the time 15 member states or the Commission.
There are also general security mechanisms against unforeseen events that can be called upon by both the member states and the applicant countries.