Information classified at the highest level of sensitivity by a government, based on an assessment that it would cause exceptionally grave damage to national security if disclosed
NSA's report on the Liberty incident was originally classified top secret.
Top secret information or activity is intended to be kept completely secret, for example in order to prevent a country's enemies from finding out about it. The top secret documents had to do with the most advanced military equipment
The designation that shall be applied only to information or material the unauthorized disclosure of which reasonably could be expected to cause exceptionally grave damage to the national security that the original classification authority is able to identify or describe