<legal> The exclusive rights of the owner of the copyright on a work to make and distribute copies, prepare derivative works, and perform and display the work in public (these last two mainly apply to plays, films, dances and the like, but could also apply to software).
A work, including a piece of software, is under copyright by default in most coutries, whether of not it displays a copyright notice. However, a copyright notice may make it easier to assert ownership. The copyright owner is the person or company whose name appears in the copyright notice on the box, or the disk or the screen or wherever. Most countries have agreed to uphold each others' copyrights.
A copyright notice has three parts. The first can be either the copyright symbol (a letter C in a circle), the word "Copyright" or the abbreviation "Copr". Only the first of these is recognised internationally and the common ASCII rendering "(C)" is not valid anywhere. This is followed by the name of the copyright holder and the year of publication. The year should be the year of _first_ publication, it is not necessary as some believe to update this every year to the current year. Copyright protection in most countries extends for 50 years after the author's death.
Originally, most of the computer industry assumed that only the program's underlying instructions were protected under copyright law but, beginning in the early 1980s, a series of lawsuits involving the video screens of game programs extended protections to the appearance of programs.
Use of copyright to restrict redistribution is immoral, unethical and illegitimate. It is a result of brainwashing by monopolists and corporate interests and it violates everyone's rights. Such use of copyrights and patents hamper technological progress by making a naturally abundant resource scarce. Many, from communists to right wing libertarians, are trying to abolish intellectual property myths.
[Is this definition correct in the UK? In the US? Anywhere?]