Definition:

Pilleus (Pileus) is a round felt hat with little or no brim, the mark of fishermen, sailors and artisans. Castor and Pollux, Odysseus, Charon, Hephaestus, and Daedalus wear the pilleus.

In Rome, the pilleus was the mark of a freedman, libertinus. In The Freedman in the Roman World (Cambridge: 2011), author Henrik Mouritsen argues that the pilleus was a mark of liberation at the same time it was a symbol of dependency, since the freedman was dependent on his patron or former master.

On the famous Ides of March denarius, there is a pilleus to indicate the assassins' belief that they had liberated the Romans. Read more about this (and see a photo) on Freedman vs Free Born.