Gap concept
Also: Sklansky gap concept
The principle that you need a stronger hand to call a raise than you would need to open the pot with a raise yourself. The gap exists because a raiser has already announced strength, while an unopened pot has announced nothing.
The idea was popularised by David Sklansky in his tournament writing and it still governs range construction. Opening from the cutoff is profitable with a wide set of hands because folds are likely; calling a cutoff open from the button with those same hands is not, because the hand you are up against is, on average, far better than a random one.
Three-betting narrows the gap by restoring the initiative, which is much of the reason modern play prefers a raise-or-fold approach to flat-calling with medium hands.
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Who came up with the gap concept?
It was popularised by the poker author David Sklansky in his tournament writing.