A government security classification of information. Short for "No Foreign National". This classification means than only native born Americans are authorized to see the NOFORN material, however access to the information is usually restricted to military personnel. In relative terms, it is less strict than "classified" or "top secret".
That page in your notes is NOFORN, don't let it leave the building.
NOFORN is a distribution (or releasability) criteria for US classified material, meaning "no foreign nationals." It is not a classification in and of itself; instead, it's used in conjunction with the actual classification of the document.
For example, a document could be classified 'Secret-- NOFORN', meaning the actual classification of the document is secret, and the information cannot be shared with foreign nationals.
Abbreviated NF. It contrasts with RELTO (releasable to, i.e. other countries).
That document is classified Secret NOFORN, better hide it before the British guy gets here!