Existing Wording
A scheduled healthcare event for a patient and/or practitioner(s) where a service may take place at a specific date/time.

Proposed Wording
A scheduled booking of a healthcare event (such as an Encounter) among patient(s) and/or practitioner(s) or other resources where the event may take place at a specific date/time.

Comments
The resource allows for appointments for other things besides patients and practitioners, e.g., locations or equipment as mentioned later. I also added "reservation for" since the appointment is not the event itself (which may end up not happening). Similar to the distinction between appointment and encounter. "Booking" may not be the best word, but I didn't want to use "appointment" to describe itself. "Reservation" could also be used instead of "booking" but booking is used elsewhere on the page.
Also, an appointment could cover multiple patients.
The "event" may not necessarily contain a "service"

Grahame's Comments
no opinion