A set of standard
values for resistors
and possibly other components as well - the E12 series has 12 values per decade
: 1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8 and 8.2. This means that a set of resistors with values between 10kOhm
and 100kOhm manufactured in E12 series will be available in the values 10kOhm, 12kOhm, 15kOhm, 18kOhm and so on.
Other, less common, series are the E24, E48 and E96. These series are almost exclusively used in very high-end applications (NASA, military, medicine etc) since most everyday applications get by just fine by using a combination of resistors from the E12 and capacitor values are often tolerant of deviation.