ac-

Multiple Entries:
  ac-    ad-  

Concise Oxford English Dictionary © 2008 Oxford University Press:
ac-/ak/

▶prefix variant spelling of ad- assimilated before c, k, and q (as in accept, acquit, and acquiesce).

Concise Oxford English Dictionary © 2008 Oxford University Press:
ad- /ad/ (also 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)

▶prefix

• 1 denoting motion or direction to: advance.

• 2 denoting reduction or change into: adulterate.

• 3 denoting addition or increase: adjunct.

– origin from L. ad ‘to’.