There is no clear legal definition for the term "invention." Turning to the law is not particularly helpful because patent law states, "The term 'invention' means invention or discovery." The term clearly does not refer to a product. Often patents describe products, but the claimed invention is a single element in that product.
An invention can be patented if it is novel. Even if something similar exists, an invention is patentable if the elements in it are not obvious. For example, cameras and cell phones are known technologies. However, the combination of the two was once not obvious. Even though cell phone cameras are similar to other cameras, they appear in a combination that was not obvious .
A product can be similar to another product without violating patent laws if elements in the first product are based upon patents that have expired. For example, there are many similar preparations of Lisinopril, a drug commonly used to treat hypertension. None of these drugs violates a patent law because the original patent for Lisinopril has long since expired. Under these conditions, the invention is in the public domain.
Similar but Different
Products can be similar without violating patent laws if the inventions that underlie both products are different. For example, computers by Apple share similarities to computers by Dell. However, the individual elements that make up an Apple are sufficiently different from the elements that make up a Dell. RAID systems provide another excellent example. A RAID system is a type of memory used in large servers. There are hundreds of different patents for RAID systems, each configured differently.