What is a "means-plus-function" claim?

Ordinarily, apparatus claim elements must be recited in precise terms describing their physical structure rather than the functions that these elements perform.  A means-plus-function claim, by contrast, allows one or more elements to be described based on their function, rather than structure.  This format is statutorily allowable according to 35 U.S.C. 112(f).   For example, instead of reciting "an alloy spring" as an element in a mousetrap, you could instead recite "a means for snapping the trap shut."  This format has several advantages.  For example, the inventor might have had in mind a steel spring, a plastic spring, a rubber spring, and dozens of other means for snapping the trap shut, where the choice of which specific one to use might have very little to do with the essence of the invention. Additionally, with a structural element, it would be fairly easy for a competitor to avoid the patent by simply coming up with a closing mechanism that the patentee had neglected to list.

In means-plus-function claims, the structure of the patentee's invention is understood from the specification of the patent. Careful:  When dealing with means-plus-function claims, the patentee has the additional burden of demonstrating that the structures of the accused product are identical to or equivalent to the structures disclosed in the patent specification.

Also note:  Although any claim element including the word "means" is presumed to be a means-plus-function element, this presumption can be rebutted if the claimed element recites structure.