A- : A, as a prefix to English words, is derived from various sources. (1) It frequently signifies on or in (from an, a forms of AS. on), denoting a state, as in afoot, on foot, abed, amiss, asleep, aground, aloft, away (AS. onweg), and analogically, ablaze, atremble, etc. (2) AS. of off, from, as in adown (AS. ofd/ne off the dun or hill). (3) AS. a- (Goth. us-, ur-, Ger. er-), usually giving an intensive force, and sometimes the sense of away, on, back, as in arise, abide, ago. (4) Old English y- or i- (corrupted from the AS. inseparable particle ge-, cognate with OHG. ga-, gi-, Goth. ga-), which, as a prefix, made no essential addition to the meaning, as in aware. (5) French a (L. ad to), as in abase, achieve. (6) L. a, ab, abs, from, as in avert. (7) Greek insep. prefix / without, or privative, not, as in abyss, atheist; akin to E. un-.

Example:
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Taxonomy
AMW, crime news, Fugitives, America's Most Wanted, Missing Persons, John Walsh, Missing Children, Captures, Michaela, behind the scenes, fbi, crime tv, fugitive watch, Search, Safety Center, history, American TV show, 20th Century Fox.

Defination from the web: