Short for aluminum/aluminium. A common, everyday name for the lightweight, corrosion-resistant metal (used in cans, foil, etc.).
飲み終わったらアルミ缶はリサイクルに出してね。
After you finish drinking, put the aluminum can out for recycling.
アルミホイルで食材を包んで焼いた。
I wrapped the food in aluminium foil and baked it.
この窓の枠はアルミ製で軽い。
The frame of this window is made of aluminium and is light.