What is the work-product rule? (Legal Ethics)

"In American civil procedure, the work-product doctrine protects materials prepared in anticipation of litigation from discovery by opposing counsel. It is also known as the work-product rule, the work-product immunity, and the work-product exception." (Craig D. Bell, Thomas E. Spahn, Christopher S. Rizek, "A Guide to the Attorney-Client Privilege and Work Product Doctrine for Tax Practitioners" (2007), p. 11.)
Under the work-product doctrine, "tangible material or its intangible equivalent" that is collected or prepared in anticipation of litigation is not discoverable, that is, the party unable to obtain the information has no other means of obtaining the information without undue hardship. For example, the witness may have left the country. Where the required showing is made, the court will still protect mental impressions of an attorney by redacting that part of the document containing the mental impressions. ("Work product". Black's Law Dictionary. p. 1298.)