Longtermism is the claim (or thesis) that we can do the most good by focusing on effects going into the longterm future:
Let strong longtermism be the thesis that in a wide class of decision situations, the option that is ex ante best is contained in a fairly small subset of options whose ex ante effects on the very long-run future are best.
So longtermism is not a general decision theory, and is only meant to be applied narrowly?
Longtermism is the claim (or thesis) that we can do the most good by focusing on effects going into the longterm future:
https://globalprioritiesinstitute.org/hilary-greaves-william-macaskill-the-case-for-strong-longtermism/