motion toward, addition to, nearness to: adsorb, adjoin, adrenal
See ad- in American Heritage Dictionary 4
prefix
ac- or af- or ag- or al- or ap- or as- or at- Toward; to. Before c, f, g, k, l, p, q, s, and t, ad- is usually assimilated to ac-, af-, ag-, ac-, al-, ap-, ac-, as-, and at-, respectively.