What security classification do you apply to information when the unauthorized disclosure of its content could reasonably be expected to cause serious damage to the national security?

In the US, according to Executive Order 13526 of December 29, 2009 - Part 1, Sec 1.2 (a)(2):

"Secret" shall be applied to information, the unauthorized disclosure of which reasonably could be expected to cause serious damage to the national security that the original classification authority is able to identify or describe.

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In the United States and Britain, data is classified as "Secret" if:

"Such material would cause "serious damage" to national security if it were publicly available."

Other nations have roughly equivalent classifications for data based on the same criteria. For a more complete list, see the attached related link.