What does "carry-on" refer to regarding airline travel?

"Carry-on" refers to items that passengers are allowed to bring with them into the cabin of the aircraft, typically subject to specific size and weight restrictions imposed by the airline. These items must often pass through security screening to ensure they comply with safety regulations. This definition encompasses personal items such as small luggage, backpacks, or any other belongings that passengers can stow in the overhead compartments or under the seat in front of them during the flight.

The other options do not accurately reflect the meaning of "carry-on." A large suitcase checked at the gate would typically be considered checked luggage, which is stored in the aircraft's cargo hold rather than the cabin. Special equipment used by flight attendants does not fall under the category of passenger carry-on items. Lastly, items stored in the cargo hold of the aircraft are not accessible to passengers during the flight, distinguishing them further from carry-on items.