Definition
A metric system prefix meaning one-tenth (1/10, or 0.1) of a base unit. Symbol: d. For example, one decimeter equals 0.1 meter, and one deciliter equals 0.1 liter.
Plain English
A prefix that shrinks a unit to one-tenth of its normal size. If you put 'deci' in front of a unit, you are talking about one-tenth of that unit.
Context Anchor
Seen in technical references, maintenance material, and unit names that use metric prefixes.
Derivation
From the Latin 'decimus,' meaning 'tenth.' The same root gives us 'decimal' (a system based on tens) and 'decimate' (originally to reduce by one-tenth). Knowing the root locks in the meaning: anything starting with 'deci' is dealing with tenths.
Why Pilots Care
Metric prefixes change the size of a measurement. Reading deci as ten instead of one tenth can make a measurement off by a factor of 100 when confused with deca.
Intuition Check
Do not confuse deci with deca. Deci means one tenth; deca means ten.
Example Sentence 1
The maintenance manual listed the fluid capacity in deciliters, so the technician converted 5 dL to 0.5 liters before servicing the system.
Example Sentence 2
Sound levels in the cockpit were checked against the decibel limit during the noise certification test.