a broad, open valley: the wide, flat lowland that lies between hills
An עֵמֶק is the low, level ground that opens out between higher land. Unlike a narrow ravine or a dry streambed, it is wide and flat enough to plow, to graze, or to draw up an army. The same word names the place where crops grow thick and where chariots roll into battle, because both happen on the same kind of ground: open bottomland.
Because these valleys are landmarks, the word becomes part of dozens of place-names. The Valley of Achor, the Valley of Rephaim, the Valley of Jezreel, the King's Valley: in each, this ordinary word for a broad lowland is doing the naming. It is one word with one picture, used both for valleys in general and for particular valleys people knew by name.
The valley's openness is what gives it its feel in the text. It is fertile and inviting, a place of grain and herds and lilies, but it is also exposed, the natural arena for a clash of armies. That double character, plenty and danger on the same flat ground, runs through the word's uses.