A pivot is a deliberate change to one or more of a startup's core hypotheses while keeping the others stable. Eric Ries catalogued ten pivot types, ranging from customer segment pivot (same product, new buyer) to platform pivot (product becomes infrastructure) to engine-of-growth pivot (changing how the business compounds).
The difference between a pivot and a 'flail' is the word 'validated'. Healthy pivots are anchored in evidence the previous direction was not going to work and a credible thesis that the new direction will. Investor patience for pivots is high when the team's logic is rigorous and low when each pivot is stylistically different.