Aliasing
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
A wave known only at its ticks
Read a sine wave nine cycles a second at ten instants a second and the readings describe a wave of one cycle a second instead. Read a wave with nothing faster than 3.7 cycles a second at more than 7.4 instants a second and the readings determine it exactly. Both facts are the same fold of the frequency axis, and it is why wagon wheels turn backwards on film.
A grid that packs the copies
Sampling a pattern on a grid makes copies of its frequencies, one for every point of a lattice, and the pattern can be read back from the samples only if the copies do not collide. For a pattern whose frequencies fill a disc the square grid wastes room between its copies. Searching every lattice of samples for the fewest that work finds a nearly hexagonal one every time — 124 samples where the square grid needs 169 — and the saving is the hexagonal packing's over the square, 13.4% in the limit. In three dimensions it is 29%, and in twenty-four the cubic grid needs 16,777,216 times as many samples as the best.
Named alongside it
The objects these essays reach for when they reach for this one.
Fourier seriesFrequencyLatticeSamplingExhaustive searchFourier transformInterpolationPeriodicitySphere-packing