A "tracer" is a tool or device that is used to track, monitor, or follow the movement or path of something, such as a bullet, a vehicle, or a digital signal. Full definition
Because the galaxies we use as tracers of the Hubble expansion will disappear. (scientificamerican.com)
Deeper redshift surveys combined with similar weak lensing maps should reveal an even greater contribution of star - forming galaxies as tracers of the matter distribution in this higher redshift range. (sciencedaily.com)
With good leads condensing those leads down based on the distress factors you can then hire a skip tracer for addresses and better yet phone numbers. (biggerpockets.com)