1. (Tennis.)ad- a prefix occurring in verbs or verbal derivatives borrowed from Latin, where it meant “toward” and indicated direction, tendency, or addition: adjoin. For variants before a following consonant, see a-5, ac-, af-, ag-, al-, an-2, ap-1, ar-, as-, at-.
Etymology: (< L ad, ad- (prep. and prefix) to, toward, at, about; c. at1)
Definition of 'ad-'
Webster Dictionary
1. ad- 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
Definitions of ad- 
