Fair use is a provision of many copyright laws that allows the unauthorized use of a copyrighted property. Fair usage is an exemption to the monopoly of distribution, usage and creation of derivative works the owner of the copyrighted property holds.
In simple terms, you can use something that's copyrighted and get away with it, but only in certain circumstances, such as for commentary, satire, teaching, criticism, or news reporting.
 In law
The trouble with fair use in law is that it is a defense rather than a right.
In the United States, the fair use exemptions are written into the U.S. Copyright Law (Title 17 of the U.S. Code), in sections 107 to 118. As with most copyright law, it can be an inconsistent legal minefield and it's best to avoid claiming fair usage unless you have first received specialist legal advice.
US Law states four factors that are considered when looking into fair use claims. These are factors to be considered by courts, lawyers and judges rather than to be a reliable guide for what is and isn't fair use:
- 1. the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
- 2. the nature of the copyrighted work;
- 3. the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
- 4. the effect of the use upon the potential market for or value of the copyrighted work.
In other words; if it comes to court, they'll probably just make it up and the side with the highest paid lawyer wins.
 Using "fair use"
Fair use must take into account several aspects, including the amount of work used, the purpose for the usage, the entity using the work and how it impacts the owner's rights. Simply asserting "fair use" while infringing the copyright of a work is not a guarantee of non-infringing use, it must still fall under the defined legal terms. Reposting a work found on another site without authorization, even if both the original site and the new site are free to use, does not guarantee a non-infringing use. For example, if Wikipedia claims "fair use" to use a picture of someone to illustrate the person's biographical article, this doesn't allow you to take that image and claim "fair use" for a completely different context, such as an article about that person's partner, or something they were involved in.
"Fair use" is a US based law and other countries will treat copyright law differently. The similarly titled "fair deal" or "fair dealing" law in the United Kingdom and other Commonwealth countries, for example is similar, but much more restrictive and less well defined. Fair dealing is primarily used when copyright infringement is purely accidental, coincidental or unavoidable. For example, if someone is making a home video of a holiday and records a song that is playing in the background, or captures an image that would otherwise be copyrighted, this would be considered fair.
 In general
As a very general, but totally unreliable guide, copyrighted materials can be considered under "fair use" in the US in the following circumstances. Usually the copyrighted work needs to satisfy more than one of these criteria so just one justification may not be enough to be legally sound.
 What it can be
- The amount of copyrighted work reproduced is small, for example, a single quote from a 2 hour film. (Though some courts have ruled that quoting an entire short work can constitute fair use.)
- The copyrighted work is directly relevant rather than incidental to the context that it's being reproduced in.
- An image is of much lower resolution and quality and/or a much smaller portion of the full, original image.
- The image is of a dead person, so there is no chance of developing any new - and copyright free - images.
- The copyrighted work is being reproduced for educational value (this is a very tedious point which is difficult to justify).
- If the work is reproduced for the sake of comment, criticism or satire/parody (the last one being another difficult point as some people's idea of satire and parody are rather lame).
 What it definitely is NOT
- "Freely available" or "widely available" does not constitute fair use.
- "Unknown copyright" does not justify fair use.
- Ignorance is, unfortunately, not a good enough excuse in a court of law.
- Posting anonymously on *Chan. You might be anonymous, but you're still a dirty thief. And probably have no friends.
 See also
- Copyright in English language teaching - a rundown of copyright law in different countries.