Relativity of simultaneity

It means two events that happen at the same time for one observer may not happen at the same time for another observer moving relative to the first. This is a consequence of Einstein's special relativity, published in 1905.

Einstein's common train-and-platform example shows that light from two strikes can reach observers differently depending on their motion. The point is not delayed eyesight, but that different inertial frames assign different times to distant events.

No. It appears in special relativity, which deals with inertial motion without needing general relativity or curved spacetime. Gravity adds more complications, but the basic simultaneity effect comes from constant relative velocity.

Both come from the Lorentz transformation in special relativity. Time dilation concerns how moving clocks tick relative to each other, while relativity of simultaneity concerns whether separated events share the same time coordinate.