A headdress is something worn on the head for show, a special day, or tradition — feathers, cloth, jewels, metal, flowers — not an everyday cap or a bicycle helmet. You see headdresses at some weddings, in dance, in old royal pictures, and in museums. The word is fairly plain and a little formal. Though actually, dress here is the old sense of covering or arranging, not a woman's dress. A crown is a kind of headdress, but people usually just say crown. A baseball cap is a hat, not a headdress. Some famous feathered headdresses belong to specific Native American nations; wearing a copy as a party costume can fail to show respect, and that is part of how people talk about the word now.