Claiming someone else's ideas or words as your own, either accidentally or on purpose.
Intellectual theft is as reprehensible as material theft.
You must acknowledge all material quoted, paraphrased, or summarized from any work. If the fact is something that can be verified in more than one reference source, such as the dates of World War II, the capitol of France, the date of James I's wedding or the name of Henry VIII's fourth wife, then there is no need to cite the source of your information. When in doubt, try to remember where you found the information. If you can remember a specific book, article, lecture, interview, or other source, then it may be that the information is not common knowledge, and a source ought to be cited. One rule of thumb is that any novel or startling assertion, or any statement that might arouse the curiosity of your reader, should be cited.