What is Boarding Pass?

Legal Definition
A boarding pass is a document provided by an airline during check-in, giving a passenger permission to board the airplane for a particular flight. As a minimum, it identifies the passenger, the flight number, and the date and scheduled time for departure. In some cases, flyers can check in online and print the boarding passes themselves. A boarding pass may be required for a passenger to enter a secure area of an airport.

Generally a passenger with an electronic ticket will only need a boarding pass. If a passenger has a paper airline ticket, that ticket (or flight coupon) may be required to be attached to the boarding pass for him or her to board the aircraft. The paper boarding pass (and ticket, if any), or portions, are sometimes collected and counted for cross-check of passenger counts by gate agents, but more frequently are scanned (via barcode or magnetic strip). The standards for bar codes and magnetic stripes on boarding passes are published by IATA. The bar code standard (BCBP) defines the 2D bar code printed on paper boarding passes or sent to mobile phones for electronic boarding passes. The magnetic stripe standard (ATB2) expired in 2010. For "connecting flights" there will be a boarding pass needed for each new flight (distinguished by a different flight number) regardless of whether a different aircraft is boarded.

Most airports and airlines have automatic readers that will verify the validity of the boarding pass at the jetway door or boarding gate. This also automatically updates the airline's database that shows the passenger has boarded and the seat is used, and that the checked baggage for that passenger may stay aboard. This speeds up the paperwork process at the gate, but requires passengers with paper tickets to check in, surrender the ticket and receive the digitized boarding pass.
-- Wikipedia
Legal Definition
A form passengers are given when boarding a transportation vehicle. Their seat number is on the pass to show its reserved for them.