iCal
In plain terms
A standard format for sharing calendar events.
Going deeper
iCalendar (iCal) defines a text format for calendar data so events and invites work across different calendar apps. It makes calendars interoperable.
Under the hood
The .ics format encodes events, recurrence, and reminders; used by Google Calendar, Outlook, and Apple.
Examples
.ics files, meeting invites, calendar sync, event sharing, Google/Outlook.
Think of it like...
Like a universal invitation card every calendar app can read.