Indicator, n. A kind of marker which, when measured or examined by a
competent observer, will reveal more macroscopic information about a
system. In ecology, certain species are very sensitive to
environmental conditions; thus their population serves as a good
indicator of the health of an ecosystem -- such as red algae.
In the early days of aerial warfare, engineers understood and
appreciated the delicate balance between armor and agility. They
devised airplanes as best they could, and then observed the results of
combat in order to make a more effective machine.
In order to accomplish this, they had a life sized picture of an
airplane. Every time an airplane came back from combat, they would
place a dot on the picture corresponding to each bullet hole. By so
doing, they hoped to discern exactly where the most damage was
sustained, and thus intelligently place armor as effectively as
It was eventually noted that there were no dots over the fuel tank.
- -- Hayward's Unabridged Dictionary