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).