What is "significant figures"

Significant \Sig*nif"i*cant\, a. [L. significans, -antis, p. pr. of significare. See Signify.]

1. Fitted or designed to signify or make known somethingl having a meaning; standing as a sign or token; expressive or suggestive; as, a significant word or sound; a significant look.

   It was well said of Plotinus, that the stars were significant, but not efficient.
   --Sir W. Raleigh.

2. Deserving to be considered; important; momentous; as, a significant event.

   Significant figures (Arith.), the figures which remain to any number, or decimal fraction, after the ciphers at the right or left are canceled. Thus, the significant figures of 25,000, or of .0025, are 25.

The significant figures of a number are digits that carry meaning contributing to its measurement resolution. This includes all digits except:

• All leading zeros;
• Trailing zeros when they are merely placeholders to indicate the scale of the number (exact rules are explained at identifying significant figures); and
• Spurious digits introduced, for example, by calculations carried out to greater precision than that of the original data, or measurements reported to a greater precision than the equipment supports.

Significance arithmetic are approximate rules for roughly maintaining significance throughout a computation. The more sophisticated scientific rules are known as propagation of uncertainty.

Numbers are often rounded to avoid reporting insignificant figures. For example, it would create false precision to express a measurement as 12.34500 kg (which has seven significant figures) if the scales only measured to the nearest gram and gave a reading of 12.345 kg (which has five significant figures). Numbers can also be rounded merely for simplicity rather than to indicate a given precision of measurement, for example to make them faster to pronounce in news broadcasts.