Online check-in is a convenient process that allows travelers to confirm their attendance for a flight or event via the internet. This service is typically available through the airline's or event organizer's website or mobile app, enabling users to select their seats, add baggage, and receive their boarding passes before arriving at the airport or venue. By using online check-in, passengers can save time and avoid long lines at the airport. Most airlines, such as Delta or American Airlines, offer this feature, which usually opens 24 to 48 hours before the scheduled departure time.