An act is rational (roughly) if it is most likely to maximize satisfaction of the agent’s preferences or utility.

Bounded rationality: the choices that people make attempt to maximize benefit and minimize cost are imperfect: humans have a hard time precisely estimating benefit and cost and thus use imperfect heuristics to pick the most promising choices. (see: Decisions under risk)

Interesting argumental ties to effective altruism, especially w.r.t. optimizing for benefit and minimizing cost.