There are two general ways that patents can be infringed: directly, which most people probably understand, and indirectly, which is fuzzy to a lot of people, even those involved with patents. A patent claim is directly infringed by some product when that product practices each “limitation” of the claim when the claim is a method claim, or includes a structure equivalent to each “limitation” of the claim when the claim is an apparatus claim, or does both in the case of a means-plus-function claim. The term “limitation” is used by lawyers and those in the know to mean the step or steps spelled out in the claim.
There is another way of directly infringing known as the doctrine of equivalents. If a product doesn’t perfectly meet the limitations of a claim, it may still infringe on the patent if it does something that a person of ordinary skill in the art (“POSITA” or “OOSITA”) would know was equivalent. While this may seem like a loophole, it’s actually quite fair. Suppose someone patents a “glass blub containing a filament that produces light when an electric current is sent through it.” The inventor may not realize that a bulb made of specially treated plastic would also work. The key to the invention isn’t the material from which the bulb is made. Thus a plastic bulb would infringe on the patent because of the doctrine of equivalents.