GG5, 中辞典, etc. Koj, Daijr/s

Better as two senses, as in the kokugos.

@@ -16 +16,5 @@
-<gloss>hills</gloss>
+<gloss>hill</gloss>
+<gloss>hillock</gloss>
+</sense>
+<sense>
+<pos>&n;</pos>

新明解国語辞典 第五版:
平地の周縁や山地の入口に位置し、似たような起伏が帯状に続いた、やや高い土地。

goo:
１ 小さな山。おか。
２ ゆるやかな起伏の低い山が続く地形。「―地帯」

goo [2] is very close to "rolling hills".
新明解 mentions "山地の入口に位置し", i.e. foothills.
These two terms will give learners a better image of the type of terrain the word refers to. 
"Hills" might should be pluralized as the word doesn't seem to refer to one single solitary hill in a vacuum (say, in the middle of a flat plain).

@@ -16 +16,3 @@
-<gloss>hill</gloss>
+<gloss>hills</gloss>
+<gloss>rolling hills</gloss>
+<gloss>foothills</gloss>