Social: interaction between people with respect to collective co-existence.
A ceremony is a ritualized social interaction between multiple participants to complete an element of work.
Anyone involved in the ceremony is considered a participant. A ceremony always has two or more participants performing two or more roles.
The Announcement Ceremony mentioned earlier has two roles—announcer and listener. Everyone within earshot who identifies as being a part of the group who is hearing an announcement is automatically a listener. The Pretakeoff Ceremony used in airplanes has two roles as well—caller and inspector. But by no means do we expect all, or even most ceremonies to have only two roles.
In the Announcement Ceremony a new team member learns their role inductively through simple repeated observation; they are more ready for the second Announcement Ceremony after observing the first one. New team members feel a special pride the first time they correctly say the preset response in sync with the rest of the team. Eventually they discover that they, too, can make an announcement to the entire team, taking on the role of announcer themselves.
Ceremony clearly delineates who is participating in a ceremony and their specific roles.
Ceremony is inherently social.