Definition
Water that has been purified by boiling it into vapor and then condensing the vapor back into liquid in a separate container, leaving behind dissolved minerals, salts, and other impurities.
Plain English
Water that has been turned into steam and then cooled back into water, so all the minerals and other dissolved substances are left behind. What you get is very pure water.
Context Anchor
Seen in aircraft maintenance instructions, especially when servicing vented lead-acid aircraft batteries.
Derivation
From the Latin 'distillare', meaning 'to drip down'. The original sense refers to the way pure droplets condense and drip from the cooled vapor — which is exactly how this water is produced.
Why Pilots Care
Using anything but distilled water in aircraft batteries introduces minerals that cause corrosion, sulfation, and early battery failure.
Analogy
It is like catching the clean water drops from steam on a cool lid while the minerals stay behind in the pot.
Intuition Check
Do not assume distilled water just means water that looks clean or is safe to drink. Here it means water that has had most minerals removed so it will not contaminate aircraft equipment.
Example Sentence 1
After checking the battery's electrolyte level, the mechanic added distilled water to bring each cell back up to the proper mark.
Example Sentence 2
Never substitute tap water for distilled water when servicing an aircraft battery.