Patents protect novel and non-obvious inventions. The scope of that protection is defined by claim terms. To understand what a patent protects, therefore, one consults the claims. But to know what the claims cover requires understanding the meaning of the claim terms. That, in turn, requires a legal determination that is often anything but straightforward.

This issue plays out every day in thousands of patent applications being examined by the Patent Office. The Patent Office applies the "Broadest Reasonable Interpretation" of claim terms to ensure that the fullest potential claim scope is properly examined. As explained by the Federal Circuit Court of Appeals in the en banc Phillips case (citing In re Am. Acad. of Sci. Tech. Ctr.), "the rules of the PTO require that application claims must 'conform to the invention as set forth in the remainder of the specification and the terms and phrases used in the claims must find clear support or antecedent basis in the description so that the meaning of the terms in the claims may be ascertainable by reference to the description.'”

More details of the process of determining claim scope at the USPTO is summarized by the flow chart below from MPEP2111. 

As explained in the MPEP, a default interpretation is based on the plain meaning - i.e., the ordinary and customary meaning given to the term by those of ordinary skill in the art. MPEP 2111.01 goes on:

The ordinary and customary meaning of a term may be evidenced by a variety of sources, including the words of the claims themselves, the specification, drawings, and prior art.

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It is also appropriate to look to how the claim term is used in the prior art, which includes prior art patents, published applications, trade publications, and dictionaries. Any meaning of a claim term taken from the prior art must be consistent with the use of the claim term in the specification and drawings.

Thus, one important source of evidence as to the meaning of a term is the prior art. This source becomes even more important when that source is the prior art cited by the Examiner. A recent Appeal decision of a Schlumberger patent application illustrates this issue.

In this case (12/996,957), the invention related to a method for determining a geometric relationship of a second well with respect to a first well. A key feature of the invention was how to process output from first and second 3-axis magnetometers positioned in the second well. The calculations included determining locations of the two magnetometers (which were both unknown) with respect to a magnetic source in the first well. The Examiner based the rejection of the invention on an interpretation of the term "magnetic source" as covering passive magnetic sources (allegedly shown in the prior art, McElhinney) under the broadest reasonable interpretation standard. The Board of Appeals initial agreed that such a broad definition was possible based on the plain meaning; however, in this case that definition was too broad because of the way the cited reference actually used the "source" term. From the decision:

So, when the issue of claim interpretation is dispositive, make sure that your proposed meaning is not only consistent with the specification, but also the prior art cited against the application.