Daniel,
A PMI-ISM is a word, definition or term that is specific the language of PMI and the PMI world. Often, these words are first created by PMI to describe a specific project management "thing" and then slowly they move into our everyday language on projects.
Here are some PMI-ISMs that I can think of:
- PMP
- Knowledge Area
- Process Groups
- Competing project constraints
- PMBOK Guide
These are all words that originated within PMI to describe some area of how PMI defines project management and they have become part of our PM language.
I believe that the word "PMI-ISM" itself was created by Rita Mulcahy to describe this multitude of words and terms that come out of PMI.