Definition
Containing no water. A substance described as anhydrous has had all water removed, or never contained any to begin with.
Plain English
Completely dry. No water in it at all.
Context Anchor
Seen in aircraft maintenance, servicing, and material specifications where moisture could affect a fluid, chemical, or component.
Derivation
From Greek 'an-' meaning 'without' and 'hydor' meaning 'water.' Literally 'without water.' Knowing this makes it easy to recognise related words like 'hydraulic' (water-powered) or 'dehydrate' (remove water).
Why Pilots Care
Some aircraft chemicals and materials must remain anhydrous to work correctly. Moisture contamination can ruin desiccants, hydraulic fluids, and certain stored chemicals, leading to corrosion or system failures.
Intuition Check
Do not read anhydrous as just “dry to the touch.” It means the material itself contains no water.
Example Sentence 1
The desiccant bag in the avionics shipping container holds anhydrous calcium chloride to absorb any moisture during transit.
Example Sentence 2
Purge lines must be supplied with anhydrous nitrogen to avoid introducing moisture into the system.