From WordNet (r) 2.0 (August 2003) [wn]:
downtown
adj : of or located in the lower part of a town, or in the
business center; "downtown Manhattan"; "delinquents
roaming the downtown streets" [ant: uptown]
n : the commercial center of a town or city [syn: {business
district}]
adv : toward or in the lower or central part of town [ant:
uptown]