parts per billion (ppb)
Parts per trillion (ppb) means "parts per billion", i.e. `10^-9` or 0.000 000 001 or also `10^-3` ppm ( parts per million). Parts per trillion is a size specification used for doping specifications of semiconductors and stability values of oscillators, among other things. For example, if an OCXO oscillator is subject to an aging of 5 ppb per year, this means that its frequency changes by `5*10^-9` per year.