1A person or thing that traces something or by which something may be traced.

1. 1.1 A bullet or shell whose course is made visible in flight by a trail of flames or smoke, used to assist in aiming.
   曳光弹
   the night sky turned into a light show of artillery, flares, and tracers

   mass noun the night was lit up by tracer

   as modifier tracer fire

   Example sentencesExamples

   • Once when we were shopping, red tracer bullets were fired from the camp over the village.
   • I dreamed of someday flying in a Spad, firing tracers from my machine guns at German barrage balloons or zeppelins.
   • The Lt said that red tracers were friendly bullets, and the Afghan's could shoot all they wanted as long as they didn't shoot at us.
   • The embedded reporter and camera see weapons fired at an unseen enemy and, if they are lucky, they may see tracers of weapons fired back.
   • The ball travelled like a tracer bullet and it was the shot of the day.
   • The origin of the bullets and tracers - the gun flashes - were almost at eye level when Rick and I looked back inside the cockpit at the altimeter.
   • A major gunnery exercise was held to expend the last of the ship's training ammunition - the tracer fire was focused on two large red inflatable targets, nicknamed the Killer Tomatoes, launched several thousand metres away from the ship.
   • He turned the bureau's news camera on the night sky to record the tracer fire now arcing impotently towards the incoming missiles.
   • After spotting the enemy, the gunners fired tracers to pinpoint the target, while the fourth helicopter opened fire with 2.75-inch rockets.
   • Tonight was the real deal; everything I'd trained for was coming to fruition, complete with tracer fire and maybe a few rockets being launched down there in the dark void of Afghanistan.
   • It was unlike any Fourth of July show you have ever seen as the sky was full of rockets, tracers and missiles.
   • Each of his 15 shots sailed 100 yards out across the fairway before bending, like a tracer bullet, back across the driving range.
   • Flares and tracer bullets sliced across the night sky after the blasts.
   • The last time I tried I kept glancing at the sky in case there were red bullet tracers, or I would suddenly jump out of bed when an American helicopter would roar across the area flying incredibly low.
   • The tracer bullets burned their way in darkness across the Tidal Basin into the White House.
   • At night the tracers became the only visible part of a fire fight: the space between combatants a mess of pink cobwebs.
   • The ramp dropped, and I clawed my way through hacking tracer fire and screaming shells to a grocery store, where I bought toothpaste and some disposable razors.
   • Then, of course, all the tracer fire started coming in, and we were really in trouble.
   • As we watched, a gunner fired again and red tracer bullets made their way slowly towards us across the river.
   • On top of this, other final elements were and filmed separately and then digitally added to the shot to replicate tracer fire, smoke, and shell casings falling from the machine guns.
2. 1.2 A substance introduced into a biological organism or other system so that its subsequent distribution can be readily followed from its color, fluorescence, radioactivity, or other distinctive property.
   示踪剂，显光剂
   they used radioactive carbon as a tracer

   mass noun transfer of tracer from spinal fluid to blood

   Example sentencesExamples

   • Tumors often use more energy than healthy tissues do and may absorb more of a radioactive tracer, which allows the tumors to appear on the scan.
   • Electrons from the tissue combine with positrons from the radioactive tracer to produce gamma rays (similar to x-rays).
   • The use of radioactive tracers makes it possible to outline a wide range of conditions, the take-up of the tracer being detected by a radioactive scanner.
   • Finally, our understanding of the trophic structure of the ecosystem was greatly advanced by following the tracer through the food web.
   • A radioactive tracer visualized the parts of the brain that were active while different types of sounds were being processed.
   • Because 210 Po associates appreciably with organic matter inside cells, unlike other particlereactive nonessential metals, it could have promise as a tracer of sinking organic matter in the ocean.
   • The distribution of tracer was followed with PETIS.
   • Using vital dyes as lineage tracers, the pattern of cleavage has been shown to be invariant but complex, particularly for the ectoderm.
   • The weak force is harnessed in modern hospitals in the form of radioactive tracers used in nuclear medicine.
   • So they enrolled him in a study, testing a new imaging system that would allow them to follow a radioactive tracer through his brain to see the plaque that has built up around the nerve cells.
   • The researchers used two different types of radioactive tracers in the same scanning sessions with 6 patients and 6 healthy controls to simultaneously monitor two different types of brain activity.
   • The node is identified by using lymphoscintigraphy and a radioactive tracer (technetium-labeled sulfur colloid or human serum albumin).
   • A spect scan can help detect cancer with the aid of cancer-seeking radioactive tracers.
   • Similar to a PET scan, the SPECT scan uses radioactive tracers to help spot cancer in your body.
   • The radioactive tracer evaluated for this study, known by the brand name Zemiva, links a fatty acid to a radioisotope which is injected in the patient.
   • Marine geochemists can exploit the distributions of these tracers to investigate physical, chemical, and biological processes operating in the world's oceans.
   • If such sites are also large enough to admit fluorescent, tracer dye molecules, the permeable areas can be visualized with an epifluorescence microscope following incubation in the tracer.
   • It relies on the difference in distribution of a radioactive tracer, such as thallium, at rest and after stress.
   • A bone scan falls under the category of nuclear medicine, which means that it uses tiny amounts of radioactive materials called tracers (radionuclides).
   • CED distributed amounts of the tracer that were comparable to the amount of drugs needed to treat brainstem diseases.
3. 1.3 A device which transmits a signal and so can be located when attached to a moving vehicle or other object.
   跟踪器（安装在移动车辆上或其他物体上，通过其发出的信号进行跟踪）
   Example sentencesExamples

   • And you'll be happy to know we knew exactly where you were, we installed tracers into all the vehicles.
   • The tracer was attached and they continued their pursuit.