The thick, cheese-like substance that is secreted from the mucous membrane covered by the foreskin in uncircumcised men. This membrane appears as lighter skin between the circumcision scar and glans penis in curcumcised men, long since rendered inoperable by the good ol' doc's buck knife all those years ago.

Side note: In my region (Dallas, TX), I have often heard smegma being referred to as duck butter. I'm not sure whether this is a hick phrase or simply part of the local vernacular.