• noun:   a device for showing the operating condition of some system
• noun:   a signal for attracting attention
• noun:   (chemistry) a substance that changes color to indicate the presence of some ion or substance; can be used to indicate the completion of a chemical reaction or (in medicine) to test for a particular reaction
• noun:   a number or ratio (a value on a scale of measurement) derived from a series of observed facts; can reveal relative changes as a function of time

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