The word Open Source is a technical term which has about a 20+ year history from 1990's. See seminal PhD Theses on Open Hardware for a historical background. The words Open Source mean more than 'something cool', which is the current popular usage of the term.
For a definition of open source software - see OSI Definition
For a definition of open source hardware - see OSHWA Definition
Open Source promotes access to a product's source materials.
Some consider open source a philosophy, others consider it a pragmatic methodology. Before the term open source became widely adopted, developers and producers used a variety of phrases to describe the concept; open source gained hold with the rise of the Internet, and the attendant need for massive retooling of the computing source code. Opening the source code enabled a self-enhancing diversity of production models, communication paths, and interactive communities. Subsequently, the new phrase "open-source software" was born to describe the environment that the new copyright, licensing, domain, and consumer issues created.
The open-source model includes the concept of concurrent yet different agendas and differing approaches in production, in contrast with more centralized models of development such as those typically used in commercial software companies. A main principle and practice of open-source software development is peer production by bartering and collaboration, with the end-product, source-material, "blueprints," and documentation available at no cost to the public. This is increasingly being applied in other fields of endeavor, such as biotechnology.
Open Source should not be confused with the Peer Production License, which does not require Open Source compliance of its products to OSHWA and OSI terms.