Flop
The first three shared cards, dealt face up for everyone at once. Before them each player sees only his own two; after them five of the seven cards are known.
The flop is three cards turned over at once that belong to everyone. Before them you have two cards and guesses; after them five of the seven are known, and the hand is either made, unfinished, or nothing. From here cards come one at a time, and no single moment changes as much again.
That is why the flop is the street of the main decision. The pot is still small and a mistake here is cheaper than on the river, but this is where the plan is chosen: whether to bet, how many streets, what size, what to do if called. A player with no plan for the turn and river on the flop pays for its absence on the turn and river.
An example from session 10
A♥Q♥ on a flop of 8♠ 4♣ J♦: nothing made, all the money in, $77.50. For that move to pay, the opponent has to fold half the time. He had A♠K♥ and did not fold. The flop said my hand had become nothing, and I heard only that it had been strong before the flop. Minus $68 in the sixth hand of the session.
The same in business
The flop is the moment the first dose of reality arrives and the model has to be rebuilt. Three months after launch, the first sales, the first response from the market. Whoever clings then to what he thought before the start is playing a preflop hand on the flop. Re-checking the model on every street is the only way not to pay for the old one on the new one.
The whole hand is in session 10: “Caution takes fewer steps, and each one costs more”.
The other terms are in the glossary.