The Open Source Initiative has created the Open Source Definition to define what open source is. Also it has set up a formal process to approve licenses to be open source. Now, does a license need this formal approval to be an open source license or is conforming with the definition enough?
"Open Source" is not a formally defined term, so there are various definitions.
Some might interpret it literally and apply it to any software whose source code is published (without necessarily any granted rights, e.g., to distribute it), some might apply it to any software that comes with certain not necessarily well-defined freedoms, some might apply it to any software that is licensed under a license that complies with The Open Source Definition, and some might apply it to any software that is licensed under a license approved by the Open Source Initiative.
According to an unsourced claim in the Wikipedia article Open-source software, OSI’s definition is "recognized as the standard or de facto definition". According to OSI itself, their founders coined the term.
And according to OSI’s FAQ, a license is only an "Open Source license" if it’s approved by the OSI:
Is <SOME LICENSE> an Open Source license, even if it is not listed on your web site?
In general, no. We run all licenses through an approval process to provide an accepted standard on which licenses are Open Source, and we list the approved ones. Be dubious of claimed Open Source-ness for licenses that haven't gone through the process. See also the license proliferation page for why this matters so much.
Another FAQ states essentially the same:
Is <SOME PROGRAM> Open Source?
Only if it uses one of the approved licenses, and releases appropriate software.
Following this definition makes sense because it objectively makes clear which licenses are Open Source licenses.
Deciding if a license complies to the OSD is not always easy, and opinions might differ. If we’d only base the decision on the OSD and ignore if the OSI approves a license or not, we would most likely end up with licenses where some agree and some disagree that it’s an Open Source license.
It depends on how strictly you interpret the term "open source".
- If you go with the strictest definition, Open Source is a term defined by the OSI so naturally OSI approval is required.
- However, as you've hinted at, the approval process is somewhat different from the Open Source Definition. Among the differences are: when OSI disapproves of a license because it is redundant, or if they haven't gotten around to approving it. You could argue, with varying success, that if OSI says it's a license is redundant with an already OSI-approved one, or if you interpret the license as conforming to the OSD so OSI will approve it someday, then it's open source.
- If you use "open source" as a generic term for software that allows unrestricted usage, modification and redistribution, then those licenses that aren't OSI-approved but approved by a similar body like the FSF or Debian Project are also open source. There are differences in opinion on what the generic term should be; FSF and Wikipedia use "free software"; others think the "free" is ambiguous with gratis, and terms like FOSS and FLOSS have been invented for this purpose.
- If you use "open source" as an even more generic term for copyright licenses that grant broad freedoms including usage, modification and redistribution, (like our site name does - we're not affiliated with the OSI! Really!) then not only are OSI or FSF approval not required, even non-software licenses will fall under this like the Creative Commons licenses. Some people refer to these licenses as simply "open", or "free".
In practice, all the popular free software licenses conform to all these definitions, so you'll never confuse anyone by saying a license is "open source", only that they may prefer you use a different term.