Embarrassing question: what does `NMU' mean?

[Eric Hanchrow <offby1@blarg.net>]

I often see this acronym in changelog files.  For example, in
/usr/share/doc/gnucash/changelog.Debian.gz, I see

    gnucash (1.2.5.cvs.20000204-1.1) frozen unstable; urgency=low

      * NMU of the unstable version to frozen, since the new upstream 
        version fixes an RC bug (fixes: #60615).

I can't figure out what it means.  I confess, I feel dumb.