The Collaborative International Dictionary of English v.0.48:

Essentially \Es*sen"tial*ly\, adv.
   In an essential manner or degree; in an indispensable degree;
   really; as, essentially different.
   [1913 Webster]

WordNet (r) 3.0 (2006):

essentially
    adv 1: in essence; at bottom or by one's (or its) very nature;
           "He is basically dishonest"; "the argument was
           essentially a technical one"; "for all his bluster he is
           in essence a shy person" [syn: basically,
           fundamentally, essentially]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "essentially":
   a outrance, absolutely, actually, all but, all in all, all out,
   almost entirely, approximately, as good as, as much as, at bottom,
   at heart, at the core, au fond, basically, beyond all bounds,
   beyond compare, beyond comparison, beyond measure, by and large,
   chiefly, completely, dead, downright, effectually, extremely,
   flat out, fundamentally, immeasurably, in essence, in itself,
   in substance, in the extreme, in the main, incalculably,
   indefinitely, infinitely, mainly, materially, most, mostly,
   on balance, on the whole, per se, perfectly, practically,
   primarily, purely, radically, really, substantially, totally,
   unconditionally, unequivocally, utterly, virtually, well-nigh,
   with a vengeance