ad-
/ad/ /əd/prefix
1Denoting motion or direction to: advance adduce
1.1Denoting reduction or change into: adapt adulterate
1.2Denoting addition, increase, or intensification: adjunct
Usage
Ad- is also found assimilated in the following forms: a- before sc, sp, st; ac- before c, k, q; af- before f; ag- before g; al- before l; an- before n; ap- before p; ar- before r; as- before s; at- before t.
Origin
From Latin ad 'to'; in the 16th century the use of ad- and its variants was extended to replace a- from a different origin such as Latin ab- (e.g. advance, from French avancer based on late Latin abante 'in front').