Outdated term for prebiotic.
In the year 1900, Tissier and Moro independently identified in France and Germany, that the microflora was different in breastfed infants and those fed by other means (Heubner et al. 1900; Tissier 1900, reviewed by Kunz 2012). As part of his PhD thesis, Tissier named these Y-shaped microorganisms "Bacillus bifidus-communis" (Rajilić-Stojanović and Vos 2014; Tissier 1900) and it was renamed in 1924 to Bifidobacterium bifidum (Rajilić-Stojanović and Vos 2014). Only in 1974 was the bifidobacterium genus recognised (Rajilić-Stojanović and Vos 2014).
Between this observation and the finding from 1926 that this unknown influence was thermoresistant, the term "bifidus factor" was coined (Schönfeld 1926). By the 1950's, the term was well established in multiple laboratories (György et al. 1954; Kristen and Petuely 1950; Levesque et al. 1959) to describe the unknown factor in breast milk that specifically supports growth of colonic bifidobacteria (Kunz et al. 2017). Today, the "bifidus factor" in breast milk is called "prebiotic" and is strongly associated with human milk oligosaccharides.