From WordNet (r) 3.0 (2006) [wn]:
lowland
adj 1: of relatively low or level country [ant: {highland(a)},
upland]
n 1: low level country [ant: highland, upland]
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]