A preferred number is one of the standard values that resistors, capacitors, and other components are manufactured in. Rather than every integer, industry uses geometric series (IEC 60063) so that a handful of values cover each decade with roughly even ratios. The common E24 series (5% tolerance) uses the bases 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91 — each then scaled by powers of ten (so 47 → 470 → 4.7 kΩ → 47 kΩ).
NumberWiki tags the integer E24 values. Coarser series (E6, E12) are subsets; finer ones (E48, E96, E192) interleave more values for tighter tolerances. The whole idea descends from the Renard numbers that Colonel Charles Renard devised in the 1870s to rationalise the sizes of balloon cables — one of engineering's quiet, ubiquitous standards.