Anticipation of invention

What does Anticipation of invention mean in American Law?

The definition of Anticipation of invention in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

One of the central concepts in the law of patents, inasmuch as the requirement of novelty, a prerequisite to valid patentability (see patentable), cannot be met if the device or process for which patent protection is sought has been “anticipated,” i.e., if the device or process, or one substantially similar, is known and in use. How similar that must be is, naturally, widely disputed and frequently litigated. It is said that the previous device must be such that a person skilled in the art could take the

prior device or process and come up with the one sought to be patented “without- adding any further inventive skill of his own, but it is not clear how much practical help that definition is either. It does appear, however, that some high degree of similarity is required. See also prior art.