Most hot dogs sold in the United States are skinless rather than natural-casing sausages.
“Skinless” does not mean the sausage was never enclosed in a casing. Manufacturers commonly force the meat mixture into a thin cellulose casing, cook the sausages, and then remove that casing before packaging. The result is a smooth hot dog without the edible natural membrane found on natural-casing varieties.
The process became commercially important in Chicago during the 1920s. Erwin O. Freund’s company developed cellulose-casing production under the name Nojax, short for “no jackets.” Skinless hot dogs became popular because they are uniform in size, generally less expensive to produce, and easy to package in large quantities.
Natural-casing hot dogs remain valued for their firmer bite and snap, so “skinless” describes casing treatment rather than freshness, curing style, or whether the sausage is smoked. Fresh, fermented, and dry-cured are different food-processing categories and do not answer this question.