In In 1998 a group of open source programmers formed an unincorporated nonprofit called the Open Source Initiative, or OSI. One of their goals was to protect the term “open source” from being misused, for example by being appropriated by a corporation. OSI wanted to somehow ensure that only software that really was open source in the traditional sense, could legally be called “open source.”
First they thought they might trademark the term “open source,” but they soon discovered that this would almost certainly be impossible. The term is too broad for the US Patent Office to ever grant anyone an exclusive right to it.
So instead they announced the OSI certification mark. Legally no one can say that their software is “OSI certified” unless it satisfies certain criteria of the Open Source Initiative Certification Program (see below). Practically this means that while anyone can call their software “open source” (or “Open Source”), you can only be really sure that it is “open source” in the original sense of the term if it is OSI certified.
To be able to call your software “OSI certified” all you have to do is license it using one of the licenses approved by OSI. The OSI board has approved these licenses according to a definition of open source they created.
You can read all about this at the OSI web site www.opensource.org.