Definition
A substance used to dilute another substance — that is, to reduce its concentration, strength, or activity by mixing it with something else. In aviation contexts, a diluent is most commonly the inert gas (typically nitrogen) added to liquid oxygen or gaseous oxygen systems to lower the oxygen concentration delivered to the user, or any fluid added to thin or weaken another fluid.
Plain English
A diluent is something you mix into another substance to make it weaker or less concentrated. Think of it as the thinning ingredient.
Context Anchor
Seen in aircraft oxygen-system descriptions, especially where a mask or regulator mixes oxygen with cabin air for breathing.
Derivation
From the Latin diluere, meaning 'to wash away' or 'to dissolve.' The same root gives us 'dilute.' A diluent is literally the thing that does the diluting.
Why Pilots Care
Correct dilution prevents both hypoxia at altitude and the discomfort of breathing undiluted oxygen at lower levels.
Intuition Check
Do not read diluent as a contaminant or impurity. In this context, it is the gas intentionally mixed with oxygen to control the breathing mixture.
Example Sentence 1
In a diluter-demand oxygen regulator, cabin air acts as the diluent, mixing with pure oxygen to provide the right concentration for the altitude being flown.
Example Sentence 2
If the diluent valve sticks closed, the pilot receives 100 percent oxygen and may experience discomfort on long flights.