Making a meeting or appointment private makes the contents of the meeting only visible to the following individuals:

• The organiser
• The attendees invited (if a meeting)
• An individual with specific permission to view private entries within your calendar (such as a Delegate). 

Anyone else who has the ability to view your calendar will see the subject of the meeting listed as 'Private Appointment' with a padlock icon against the calendar entry.

1) In the calendar of classic Outlook, select the meeting or appointment you want to make private.

2) In the Tags group, select Private.

3) Select Save & Close. In calendar view, a lock symbol will appear on the event to indicate it is private.