A patent is a set of exclusive rights granted by a state to an inventor or his assignee for a limited period of time in exchange for a disclosure of an invention.
The term patent usually refers to a right granted to anyone who invents or discovers any new and useful process, machine, article of manufacture, or composition of matter, or any new and useful improvement thereof. The additional qualification utility patent is used in the United States to distinguish it from other types of patents (e.g. design patents) but should not be confused with utility models granted by other countries. Examples of particular species of patents for inventions include biological patents, business method patents, chemical patents and software patents.
Some other types of intellectual property rights are referred to as patents in some jurisdictions: industrial design rights are called design patents in some jurisdictions (they protect the visual design of objects that are not purely utilitarian), plant breeders' rights are sometimes called plant patents, and utility models or Gebrauchsmuster are sometimes called petty patents or innovation patents. This article relates primarily to the patent for an invention, although so-called petty patents and utility models may also be granted for inventions.
Certain grants made by the monarch in pursuance of the royal prerogative were sometimes called letters patent, which was a government notice to the public of a grant of an exclusive right to ownership and possession. These were often grants of a patent-like monopoly and predate the modern origins of the patent system. For other uses of the term patent, which were land grants by early state governments in the USA. This reflects the original meaning of letters patent that had a broader scope than current usage.
More at : http://en.wikipedia.org/wiki/Patent