It is language change that occurs from social, cognitive, or physiological pressures from within the system.
[2] These linguistic changes enter language primarily through the vernacular and spread throughout the community without speakers' conscious awareness.
New linguistic changes that enter the language from below are most commonly used by the interior socioeconomic classes, as displayed by William Labov's curvilinear principle.
However, forms that have overt prestige are more prized by these groups, so when changes from below rise to the level of awareness, they are frequently stigmatized and rejected by the very people using them.
The changes made by individuals such as these, who are upwardly mobile and intentionally nonconformist, then diffuse into the speech of broader groups as described by Bill Labov’s Constructive Nonconformity Principle.