In copyright law, attribution is the requirement that an author be given credit for their work in any context in which it is used. It is required by most copyright and copyleft licenses, such as GNUFDL and CC-by.
Attribution is often considered the most basic of requirements made by a license, as it allows an author to accumulate a positive reputation that partially repays their work and prevents others from claiming fraudulently to have produced the work.
Typically, the only way a person may copy content without providing proper attribution is to ask for explicit permission. Not even a requirement of attribution may be placed on a work in the public domain, since the creator has given up ownership of the work.
Attribution is also a psychological concept. When people watch the world, they do not see it as a completely random stream of happenings, but tend to attribute meanings to things. So, for example when you see someone fall over, you can attribute this to a stable trait ("clumsiness"), or to a feature of the situation ("banana peel on the floor"), or to random chance ("just one of those crazy flukes"). There are a number of theories about the orderly ways in which people make these attributions (see attribution theory), and there are a number of well documented quirks in the way people make them (see attributional bias, and fundamental attribution error).