ferment v. foment

The literal senses of ferment and foment are easily distinguished. Fermentation is the anaerobic conversion of carbohydrates to alcohol and carbon dioxide by yeast or bacteria. Fomentation is the therapeutic application of warmth and moisture.

The figurative senses are a bit trickier. As a noun ferment means ‘a state of unrest’. As an intransitive verb, it means ‘to be in a state of agitation’: ‘A dispute began to ferment.’ As a transitive verb, it means ‘to excite into a state of agitation’. Foment has the similar transitive meanings of ‘goad’, ‘instigate’, or ‘provoke’: ‘to foment rebellion’. Ferment can carry either a positive or a negative connotation, but foment is almost always negative.

Madrigal Communications on ferment v. foment