Definition
A number system based on ten, using the digits 0 through 9. The position of each digit determines its value, with each place to the left representing ten times the value of the place to its right (ones, tens, hundreds, thousands, and so on). Fractional values are shown to the right of a decimal point, with each place representing one-tenth the value of the place to its left.
Plain English
The everyday number system we use, built on tens. The position of each digit tells you what it's worth — ones, tens, hundreds, and so on — and digits after the decimal point handle anything smaller than one.
Context Anchor
Seen in aviation calculations, aircraft manuals, charts, maintenance measurements, and performance planning when numbers are written with whole numbers or decimal points.
Derivation
From the Latin 'decimus,' meaning 'tenth.' The system is called decimal because it is built around the number ten — likely because humans have ten fingers, which made counting in tens natural from earliest times.
Why Pilots Care
Pilots must read and use numbers exactly. A small mistake with a decimal point can change a fuel amount, distance, weight, or measurement enough to affect safety.
Intuition Check
Decimal does not only mean “the numbers after the dot.” Here it means the whole base-ten way of writing numbers, whether or not a decimal point is present.
Example Sentence 1
Aviation radio frequencies like 122.75 MHz are written in the decimal number system, with the digits after the point showing fractions of a megahertz.
Example Sentence 2
The altimeter setting of 30.12 is entered in the decimal number system.