In longtermism, a trajectory change is a persistent change to total value at every point in the long-term future. Trajectory changes have also been described as slight or significant changes to "the world's development trajectory, or just trajectory for short", with that referring to:^[1]

a rough summary way the future will unfold over time. The summary includes various facts about the world that matter from a macro perspective, such as how rich people are, what technologies are available, how happy people are, how developed our science and culture is along various dimensions, and how well things are going all-things-considered at different points of time. It may help to think of the trajectory as a collection of graphs, where each graph in the collection has time on the x-axis and one of these other variables on the y-axis.

[...] If we ever prevent an existential catastrophe, that would be an extreme example of a trajectory change. There may also be smaller trajectory changes. For example, if some species of dolphins that we really loved were destroyed, that would be a much smaller trajectory change.

Most longtermist interventions focus on trajectory changes, including but not limited to existential risk reduction. However, some longtermist interventions focus on alternative objectives, such as speeding up development.