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  1. a prefix occurring in loanwords from Latin, where it meant “toward” and indicated direction, tendency, or addition: adjoin. Usually assimilated to the following consonant; see a-5, ac-, af-, ag-, al-, an-2, ap-1, ar-, as-, at-.

Origin of ad-

< Latin ad, ad- (preposition and prefix) to, toward, at, about; cognate with at1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for ad-

ad-

prefix
  1. to; towardsadsorb; adverb
  2. near; next toadrenal

Word Origin

from Latin: to, towards. As a prefix in words of Latin origin, ad- became ac-, af-, ag-, al-, an-, acq-, ar-, as-, and at- before c, f, g, l, n, q, r, s, and t, and became a- before gn, sc, sp, st
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for ad-

word-forming element expressing direction toward or in addition to, from Latin ad "to, toward" in space or time; "with regard to, in relation to," as a prefix, sometimes merely emphatic, from PIE *ad- "to, near, at" (cognate with Old English æt; see at). Simplified to a- before sc-, sp- and st-; modified to ac- before many consonants and then re-spelled af-, ag-, al-, etc., in conformity with the following consonant (e.g. affection, aggression). In Old French, reduced to a- in all cases (an evolution already underway in Merovingian Latin), but written forms were refashioned after Latin in 14c. in French and 15c. in English words picked up from Old French. In many cases pronunciation followed the shift.

Online Etymology Dictionary, © 2010 Douglas Harper

ad- in Medicine

ad-

pref.
  1. 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:adductor, acclimation, agglutinant.
  2. Near; at:adrenal.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.