1. One that indicates, especially:
a. A pointer or an index.
b. An instrument used to monitor the operation or condition of an engine, furnace, electrical network, reservoir, or other physical system; a meter or gauge.
c. The needle, dial, or other registering device on such an instrument.
2. Chemistry Any of various substances, such as litmus or phenolphthalein, that indicate the presence, absence, or concentration of another substance or the degree of reaction between two or more substances by means of a characteristic change, especially in color.
3. Ecology An indicator species.
4. Any of various statistical values that together provide an indication of the condition or direction of the economy.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.