1. A basket filled with earth and stones, used in building fortifications.
2. A metal cage filled with rocks, concrete, sand, stones, etc., and used in constructing dams, embankments, and other structures.
Etymology: Italian gabbione, big cage
There are many example pictures on Wikipedia.
I found the word at a permaculture site, while trying to learn more about creating microclimates (I just bought some Prunus mume, which are not specifically hardy here).
This site talks about gabions being filled with construction debris (and is the source of the below picture). Interesting idea...I could clean out the garage.