bal·last
noun/ˈbæləst/
/ˈbæləst/
[uncountable]- heavy material placed in a ship or hot-air balloon to make it heavier and keep it steady
（船中保持平衡的）压载；（热气球的）镇重物 - a layer of stones that makes a strong base on which a road, railway, etc. can be built
（用作公路或铁路路基的）道砟
Word Originmid 16th cent.: probably of Low German or Scandinavian origin.