Patentability

Within the context of a national or multilateral body of law, an invention is patentable if it meets the relevant legal conditions to be granted a patent. By extension, patentability also refers to the substantive conditions that must be met for a patent to be held valid.
Requirements.
The patent laws usually require that, for an invention to be patentable, it must be:
Usually the term "patentability" only refers to "substantive" conditions, and does not refer to formal conditions such as the "sufficiency of disclosure", the "unity of invention" or the "best mode requirement".
Judging patentability is one aspect of the official examination of a patent application performed by a patent examiner and may be tested in post-grant patent litigation.
Prior to filing a patent application, inventors sometimes obtain a patentability opinion from a patent agent or patent attorney regarding whether an invention satisfies the substantive conditions of patentability.
Opposition and reexamination.
Many national and regional patent offices provide procedures for reconsidering whether or not a given patent is valid after grant. Under the European Patent Convention, any person can file an opposition provided they act promptly after grant of the patent. In the United States, members of the public can initiate reexamination proceedings. Japan provides similar options as well. In India, the Patent Act provides for a dual opposition system i.e. pre-grant opposition as well as post grant opposition. While a pre-grant opposition may be filed by any person, the post grant opposition may only be filed by a person interested in the field of invention.
Members of the public can also initiate lawsuits in the courts of various nations to have patents declared invalid.
United Kingdom patents can be reviewed by way of a non-binding opinion issued by the Patent Office, or by formal applications for revocation before the Patent Office or the Court. If the patent survives a revocation action, this is noted for future reference by way of a Certificate of contested validity.
Infringement.
The fact that an invention is patentable or even patented does not necessarily mean that use of the invention would not also infringe another patent. The first patent in a given area might include a broad claim covering a general inventive concept if there is at that point no relevant prior art. Later, a specific implementation of that concept might be patentable if it is not disclosed in the earlier patent (or any intervening prior art), but nevertheless still falls within the scope of the earlier claim (covering the general concept). The later inventor must, therefore, obtain a license from the earlier inventor to be able to exploit their invention. At the same time, the earlier inventor might want to obtain a license from the later inventor, particularly if the later invention represents a significant improvement in the implementation of the original broad concept. In this case, the two enter into a cross license.
Thomas Edison's thin carbon filament light bulb was a patentable improvement over the earlier patented Woodward and Evans thick carbon filament light bulb. Thomas Edison bought the Woodward patent for $5,000 US before he began his development work so that Woodward would not be able to sue him for patent infringement after Edison became commercially successful.
National laws.
United States.
Under United States patent law, inventorship is also regarded as a patentability criterion. It is a constitutional requirement. Congress's ability to grant patents is authorized only for the inventor. This was confirmed by case law: "Inventorship is indeed relevant to patentability under 35 U.S.C. § 102(f), and patents have in the past been held unenforceable for failure to correctly name inventors in cases where the named inventors acted in bad faith or with deceptive intent."
Details on patentability in the U.S. can be found in the Manual of Patent Examining Procedure or MPEP. This is published by the United States Patent and Trademark Office (USPTO) and is the reference manual used by both patent examiners and patent agents/attorneys. Chapter 2100, in particular, gives a comprehensive overview of the standards for patentability, a discussion of the related case law, and guidance on how to overcome an examiner's rejection of a given set of claims.
In the United States, the patent grant is presumptive, e.g. a patent shall issue unless the patent statutes preclude the grant. In other words, the burden is on the Patent Office to prove why a patent should not be granted. Once a patent issues, however, it is presumed valid and a court may declare it invalid only on the basis of clear and convincing evidence.
Europe.
Terminology in Europe (the member states of the European Patent Organisation) is slightly different. While in the U.S. all patent applications are considered to cover inventions automatically, in Europe a patent application is first submitted to a test whether it covers an invention at all: the first out of four tests of (the other three being novelty, inventive step, and industrial applicability). So an "invention" in European legal terminology is similar to "patentable subject-matter" in the American system.
India.
Under the Indian Patent Act (1970), "inventions" are defined as a new product or process involving an inventive step and capable of industrial application. Thus the patentability criteria largely involves novelty, inventive step and industrial application or usability of the invention. In addition, section 3 of the Patent Act, 1970, also provides a list of non-patentable inventions for e.g. inventions that are frivolous or contrary to well established to natural laws.