The term BI Semanitc model has been bounded around a lot over the last year. I have come across a few really good articles which explain this in more depth. The definition is that the BI Semantic Model is one model for all end user experiences.
Here are my a few of articles that clearly explain this term with a really helpful diagram.
Updated: Analysis Services – Roadmap for SQL Server Code Name “Denali” and Beyond
So, what is the BI Semantic Model?
Understanding the SQL Server 2012 BI Semantic Model (BISM)