Longtermism is the view that what matters most about our actions is their impact over the long-term future.
When judging the value of an action, future considerations may be highly relevant. Ethical theories according to which future people are moral patients, whose wellbeing is of comparable importance to the wellbeing of present-day people, imply that we ought to consider the long-term future of humanity, rather than just the next hundred years or so.
In fact, since it is possible that nearly everyone who will ever live has not yet been born, these ethical theories tend to imply that if there is anything that can be done to positively shape the long-term future, then this will be more valuable than any other possible action.
On the other hand, if it is impossible to influence the long-term future—for instance, if humans are doomed to go extinct within the next century, or if we simply lack sufficient foresight, then one should focus on the short- and medium-term impact of one’s actions, regardless of one’s ethical views on future potential people.
Centre for Effective Altruism. Cause profile: The Long-Term Future
Beckstead, Nick. 2013. On the overwhelming importance of shaping the far future.
A philosophy dissertation that is likely the most thorough treatment of the subject.
Todd, Benjamin. 2013. How important are future generations?
An examination of the decision-relevance of the long-term future.