An eyewitness, on a basic level, is somebody who "witnesses" something with their "eyes". On a more complex level, it is somebody who "witnesses", as in, observes an event, "something", i.e. an event, generally an important event, with their...um..."eyes." On an even more complex level - actually, to be perfectly honest, that's about as complicated as it gets. There, um, isn't really much more to say in terms of definition. That kind of covers it actually.)
OMG YOU FOUND IT!
But you should still carry on reading. You wouldn't want to miss anything interesting, now, would you?
Example of Use of An Eyewitness
On July 22nd, 2008, in Brooklyn, New York City, at 11:06pm local time, a late night convenience store was robbed. A man walked into the shop with his hood up, forced a gun in the face of Mr. Greenberg, the owner, and demanded he empty the till. Mr. Greenberg did empty the till, and then was shocked to see the man run away with the contents. "Not even a please, or thank you, or anything!" he said. Naturally, the whole of New York was in riotous uproar, unable to believe that such an event could possibly be allowed to happen without the interference of Spiderman or the NYPD. At the very least the Law & Order squad could have got involved. The owners, Mr. and Mrs Johann Greenberg, appeared on national TV asking if anyone had seen anything that night. Quite a few people replied saying that they had seen many things, thank you very much, in fact one had even listed everything important he'd seen: a man walking a dog, a half-eaten donut on the floor and the seat of a park bench. However, only one person had seen anything related to the robbery.
The eyewitness, who wished to remain anonymous, had been walking past the shop when the rude intruder entered. He used his eyes to witness the attack, and was able to take a mental photograph of the attacker (male, young, Nike hoody, gun) which helped the police narrow down the list of suspects to 2,756,630. However, the eyewitness also managed to witness a very important detail - the words "Property of Mrs Doris Goldman, please return to Flat 96, East 131 St" written on the side of the gun. Four months later, the police told the Greenbergs that they knew who had committed the crime - Mrs Doris Goldman, an 82-year-old. They even knew where she lived.
Doris Goldman protested her innocence throughout the trial, stating it was her young, male, Nike-wearing next door neighbour who had stolen her gun and committed the crime. The jury, however, found her guilty, and she was imprisoned for 9 months for the theft of $4.58.
Following this, the eyewitness was placed into eyewitness protection for his safety, in case members of Doris's extended family came to attack him. Fortunately, none did, not even her scary first husband, so the protection was removed. 
The US Government is famed for its amazing treatment of eyewitnesses who need protection. Here is the Eyewitness Protection Act Guidelines in full:
EYEWITNESS PROTECTION MEASURES: IN FULL
In association with Bosch Toast-A-Matic Inventions Ltd., 2009
Compiled by Col. John Porter
Here is a comprehensive list of all the measures taken to ensure that all eyewitnesses of severe crimes are kept safe from such problems as angry, vindictive murderers, revenge-seeking thugs and faulty toasters.
- Relocation – all eyewitnesses will be moved to a secret location somewhere in New York (231 East 73rd Street). The relocation from their home address makes it a lot more difficult for potential attackers to gain access to the witnesses. The move will not be permanent. However, in the interests of making it easy for the police to find the eyewitnesses easily when they need to talk to them, this location will be marked on all maps and Sat Navs as “Eyewitness Protection Service – Relocation HQ." There will also be a plaque above the door.
- New Identity – all eyewitnesses will be provided with a new identity, free of charge. Depending on the severity, passports, bank details and entire personal histories may be rewritten. To avoid confusion, however, all names of the eyewitnesses will be kept the same bar a few letters – i.e. Peter Pakrinov would become Petey Pakrinof, and George W. Bush would become George V. Bush. This is thought to be sufficient deception to fool moderate would-be criminals, such as, um, The Mafia.
- Makeover – all eyewitnesses will receive a complementary government-approved haircut. This change of appearance should help protect the innocent eyewitness from violence. (The haircuts are in the style of your favourite President!)
- Toaster – all eyewitnesses will receive a new Bosch Toaster 3000! It is unclear how this will be able to protect the eyewitnesses, but it sure does make toast that bread mighty fine!
END OF DOCUMENT.
These simply marvellous preparations for the protection of eyewitnesses have, in the USA in particular, led to the development of a new generation of citizens whose only career aim is to actually become an eyewitness. The perks are obvious - no rent, leaving the family behind, moving to New York, getting a new toaster! With this array of offers available, it is unsurprising that so many turn to eyewitness work as their future profession. The UK exam board OCR is currently considering bringing in a "Witness Studies" A Level, as a suitable qualification to provide training for those who wish to continue this career path.
How Do I Become An Eyewitness And Receive This Protection?
Many people want to be eyewitnesses and receive protection (and toasters). In fact, over 95% of children aim to become either a footballer, singer or eyewitness. However, becoming an eyewitness is not as easy as it sounds. First all, you have to have to have two eyes. This is vital. One-eyed witnesses often get thrown out of court, especially when they enter in the middle of court proceedings, asking for directions to the library. Throughout the history of the US courts there have been no one-eyed eyewitnesses at all, and very few in British history (though the Guardian has reported about three eyed witnesses in the Princess Diana case, in all the other national newspapers it was reported as "three eye witnesses", and so The Guardian probably just had a typing error).
Secondly, you need to position yourself somewhere where crime is likely to happen. For example, the USA, or China. Then you have to wait, with your eyes peeled. Do not literally peel your eyes, this is painful and a waste of your time (plus the eye peel tastes disgusting, even in cakes). Simply sit, and watch. You may get bored. In fact you are very likely to get bored. Only professional eyewitnesses can keep their eyes peeled for long periods of time, such as years. You, however, as a novice, will probably give up after an hour, so just go and try something else if you are not good enough for a professional eyewitness. Try being a footballer, its way easier.
Witness Protection Myth
98% of eyewitnesses are referred to in the eyewitness business as "opportunistic eyewitnesses" - they witness with their eyes only randomly out of the blue, without even planning for it. They have not endured the long hours of sitting and watching, yet they still reap the glory and protection. These people are the envy of professional eyewitnesses, and are often hunted down and murdered by other eyewitnesses. This has, in fact, led partly to the increased security and necessity for eyewitness protection. Most of the public believe the Witness Protection Act was put in place to protect harmless eyewitnesses against the murderers who committed the murders they witnessed with their eyes. However, the Witness Protection Act more often than not protects opportunistic eyewitnesses from professional eyewitnesses. 
- (Coincidentally, however, the eyewitness in question actually died in a tragic accident the very next day after the witness protection was removed. The accident involved a carving knife to the head.)
- UNLESS THEY KNOW SOMETHING WE DON'T...
- Also, try saying those last two sentences five times fast backwards.