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From Biology-Online Dictionary
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1. An element or compound containing atoms that can be distinguished from their normal counterparts by physical means (e.g., radioactivity assay or mass spectrography) and can thus be used to follow (trace) the metabolism of the normal substances.

2. A coloured substance (e.g., a dye) used as a tracer to follow the flow of water.

3. An instrument used in dissecting out nerves and blood vessels.

4. A mechanical device with a marking point attached to one jaw and a graph plate or tracing plate attached to the other jaw; used to record the direction and extent of movements of the mandible.

See: tracing.

Origin: M.E. Track, fr. O. Fr. Tracier, to make one's way, fr. L. Traho, pp. Tractum, to draw, _ -er, agent suffix