adherence n.

word-forming element expressing direction toward or in addition to, from Latin ad "to, toward" in space or time; "with regard to, in relation to," as a prefix, sometimes merely emphatic, from PIE root *ad- "to, near, at."

Simplified to a- before sc-, sp- and st-; modified to ac- before many consonants and then re-spelled af-, ag-, al-, etc., in conformity with the following consonant (as in affection, aggression). Also compare ap- (1).

In Old French, reduced to a- in all cases (an evolution already underway in Merovingian Latin), but French refashioned its written forms on the Latin model in 14c., and English did likewise 15c. in words it had picked up from Old French. In many cases pronunciation followed the shift.

Over-correction at the end of the Middle Ages in French and then English "restored" the -d- or a doubled consonant to some words that never had it (accursed, afford). The process went further in England than in France (where the vernacular sometimes resisted the pedantic), resulting in English adjourn, advance, address, advertisement (Modern French ajourner, avancer, adresser, avertissement). In modern word-formation sometimes ad- and ab- are regarded as opposites, but this was not in classical Latin.

c. 1400, from Old French hesitacion or directly from Latin haesitationem (nominative haesitatio) "a hesitation, stammering," figuratively "irresolution, uncertainty," noun of action from past participle stem of haesitare "stick fast, remain fixed; stammer in speech," figuratively "hesitate, be irresolute, be at a loss, be undecided," frequentative of haerere (past participle haesus, first person perfect indicative haesi) "to adhere, stick, cling."

This is said by Watkins to be from PIE root *ghais- "to adhere, hesitate" (source also of Lithuanian gaišti "to delay, tarry, be slow"), but some linguists reject the proposed connection; de Vaan offers no etymology.

1620s, "act or state of sticking or being stuck, a being united or attached," from French adhésion or directly from Latin adhaesionem (nominative adhaesio) "a sticking to," noun of action from past-participle stem of adhaerare "to stick to, cling to," from ad "to" (see ad-) + haerere "to stick" (see hesitation). The earliest English use is of persons ("faith is adhesion unto God"), but by 18c. adhesion was "generally used in the material, and adherence in the metaphysical sense." [Johnson]