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: Patentable subject matter, i.e., a kind of subject-matter eligible for patent protection Novel (i.e. at least some aspect of it must be new) Non-obvious (in United States patent law) or involve an inventive step (in European patent law) Useful (in U.S. patent law) or be susceptible of industrial application (in European patent law) 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 the patent is granted. In the United States, members of the public can initiate reexamination proceedings. Japan provides similar options. 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 fact 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