from The American Heritage® Dictionary of the English Language, 4th Edition
- 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.
- Near; at: adrenal.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- near, at.
- toward, to, tendency, or addition.
from the GNU version of the Collaborative International Dictionary of English
- As a prefix ad- assumes the forms ac-, af-, ag-, al-, an-, ap-, ar-, as-, at-, assimilating the d with the first letter of the word to which ad- is prefixed. It remains unchanged before vowels, and before d, h, j, m, v. Examples: adduce, adhere, adjacent, admit, advent, accord, affect, aggregate, allude, annex, appear, etc. It becomes ac- before qu, as in acquiesce.
Latin, from ad, to; see ad- in Indo-European roots.(American Heritage® Dictionary of the English Language, Fourth Edition)
from the Latin prefix ad-. (Wiktionary)