Definition
The digits in a number that carry real measurement meaning — that is, the digits known to be reliable plus one final digit that is estimated. Leading zeros that only mark the decimal place are not significant; non-zero digits and zeros between them are. Significant digits indicate the precision of a measurement, not its magnitude.
Plain English
The digits in a number that actually tell you how precise a measurement is. Zeros at the front (like the 0 in 0.025) just hold the decimal point and don't count; the meaningful digits are the ones that follow.
Context Anchor
Seen in weight-and-balance work, performance calculations, maintenance measurements, and any place a pilot or technician rounds numbers from charts, gauges, or calculations.
Derivation
From Latin significare, 'to signify' or 'to mean something.' The digits that actually mean something — that carry information about the measurement — as opposed to digits that are just placeholders.
Why Pilots Care
Using the correct number of significant digits prevents rounding errors that could affect safety margins in fuel, weight, or altitude computations.
Analogy
If a ruler is marked only in inches, you should not claim a length as 10.237 inches. The extra digits make the measurement look more exact than the ruler allows.
Intuition Check
Significant does not mean “important” in the everyday sense. Here it means “digits justified by the accuracy of the measurement or calculation.”
Example Sentence 1
The torque value of 25.0 ft-lb has three significant digits, while 25 ft-lb has only two.
Example Sentence 2
When converting indicated airspeed to true airspeed, the crew kept only three significant digits to match the accuracy of the original instrument reading.