A product of the intellect that has commercial value, including copyrighted property such as literary or artistic works, and ideational property, such as patents, appellations of origin, business methods, and industrial processes.
Here's some history:
By the time the U.S. Constitution was being written, the Founding Fathers had a good philosophical grasp of intellectual property, and provisions regarding it were included: Article I, Section 8 gave Congress the power "To promote the progress of Science and Useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries." Almost as soon as the Constitution was ratified, Congress proceeded to set up a patent law to protect monopolies on inventions. By 1836 the U.S. Patent Office was functioning effectively, and it soon became apparent that invention fared better under the U.S. system than under any other. A monopoly for a limited time on a specific invention encouraged people to invent, knowing they could be protected, and enabled them to sell rights to others who had the capital or existing trade to manufacture and promote an invention.