Definition
A liquid, usually distilled water, added to a concentrated acid to lower its strength to a desired level for use in aircraft batteries or other servicing applications. The diluent is added to the acid (not the other way around) to control the heat and splashing produced by the chemical reaction.
Plain English
Something — almost always pure water — that is mixed with a strong acid to weaken it before it is used.
Context Anchor
Seen in aircraft battery maintenance, especially when discussing lead-acid battery liquid and battery servicing procedures.
Derivation
From the Latin diluere, meaning 'to wash away' or 'to thin down.' A diluent is simply something that thins another substance — in this case, an acid.
Why Pilots Care
Mishandling concentrated acid during battery servicing can cause severe burns, damaged equipment, and ruined batteries. Knowing what a diluent is — and the rule of always adding acid to water rather than water to acid — is a basic safety point in maintenance work.
Intuition Check
Do not read diluent as neutralizer. An acid diluent weakens the acid mixture, but the liquid can still be dangerous and corrosive.
Example Sentence 1
The technician carefully added the acid to the diluent in the battery shop, never the other way around.
Example Sentence 2
Using the wrong ratio of acid diluent can damage the battery plates and reduce service life.