Is it rude to recline your airplane seat?

The seat reclines because the airline built it that way and sold you the function, so using it is within your rights, and the etiquette obligation is about how rather than whether: glance back before reclining, do it slowly, and return upright during meal service. Reclining fully during a two-hour daytime flight while the person behind is working on a laptop is inconsiderate; reclining on an overnight transatlantic flight is entirely normal and expected. The passenger behind also has no right to physically block the seat or bang on it, and the devices sold to jam seatbacks are banned by most airlines for good reason.

The honest framing is that this is a manufactured conflict. Average seat pitch on US carriers has shrunk substantially over decades, from roughly 34 or 35 inches to as little as 28 to 31 on some aircraft, while passengers have gotten larger, so the recline that was once a minor adjustment now directly consumes the space of the person behind. That turns a design decision into a passenger-versus-passenger dispute, which is convenient for everyone except the passengers. Airlines have quietly resolved it in places by installing pre-reclined non-reclining seats on short-haul aircraft. Until then, ask, recline gently, and direct the actual irritation at the seat map rather than at the stranger in front of you.