From WordNet (r) 2.0 (August 2003) [wn]:
lowland
adj : of relatively low or level country [ant: upland]
n : low level country [ant: highland]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Lowland \Low"land\, n.
Land which is low with respect to the neighboring country; a
low or level country; -- opposed to highland.
[1913 Webster]
The Lowlands, Belgium and Holland; the Netherlands; also,
the southern part of Scotland.
[1913 Webster]