Here's how the United States Patent and Trademark Office (USPTO) defines a "patent":
This outstanding video, entitled "The Patent Process: An Overview for Jurors" was produced by the Federal government to introduce key patent concepts, terms and legal frameworks to jurors in patent cases.
As an inventor, your first step - before you disclose your invention to anyone else - is to file a provisional patent application. The process is relatively inexpensive, and can be done by the inventor without hiring a patent attorney or patent agent. One of the best books on how to write and file a provisional patent application yourself is this one: