Updated: Oct 23, 2020
Who is listed as an inventor on your patent can affect its validity. An incorrectly named inventor may make your patent invalid.
Definition of Inventor
Who is an inventor of a patent is not the same as who is an owner of a patent. Therefore, one can be an inventor, but not an owner. Likewise, one can be an owner, but not an inventor. However, one can be both an inventor and owner.
An inventor is one who contributed to the conception of the invention. The "invention" is defined as at least one of the patent "claims" in the patent. "The patent claims are the numbered paragraphs (1, 2, 3, etc.) that are at the end of the patent and often followed by the phrase "I claim" or "We claim".
Conception" is the creation of the invention. A test for conception is whether the person had an idea that was "definite and permanent" enough so that one skilled in the art of the invention could understand the invention.
In conceiving the invention, one may consider and adopt the ideas of others, including the suggestions of an employee or consultant. That person can still be an inventor, even if the suggestion turns out to be the key to solving the problem addressed by the invention.
Those who merely carry out testing of the invention would not be inventors. And those who merely help write the patent would not be inventors.
Conception is different from something referred to as "reduction to practice". The latter is the finalization of the invention to ensure that the invention works for its intended purpose.
Reduction to practice can be in the form of a working prototype. It can also be in the form of filing a patent application. However, to be an inventor, one does not have to reduce the invention to practice.
An invention can be created by two or more persons. They need not physically work together or work at the same time. They need not contribute to the invention in the same type or amount. And they do not need to contribute to the conception of every patent claim.