STRIPS
Fikes and Nilsson described a planner in which an action is given by preconditions and by lists of what it adds to and deletes from the state of the world.
Why it matters
Planning became computable: instead of proving a theorem about the new state, the machine simply edits the description of the world.
The earlier approach required proving a theorem for every step, and the number of axioms about what an action does not change grew without bound. STRIPS sidestepped that by assumption: whatever is not in the lists stays as it was. That representation of an action still underlies the PDDL language and every modern planner. The price was giving up a complete logical description.