http://www.objectmatter.com/glossary.htm#IMPEDANCEMISMATCH = Java vs SQL (RDBMS) -- "Java developers working on applications that access relational databases are forced to write massive conversion routines using two different languages, SQL and Java." -- "Solved" by their "Visual Business Sight Framework" (VBSF) product. ;->
Unfortunately, the article and the author suffer from impedance mismatches themselves. Readers beware and learn to distrust the "impedance mismatch" kind of people. The profound root of the issue is a severe ObjectRelationalPsychologicalMismatch, which basically says that many "object gurus" are complacent in a very superficial understanding (if it can be called that way) of relational databases, and on the larger scope of what ComputingScience is about. Illustrative from that article:
Why does a technological impedance mismatch exist? The object-oriented paradigm is based on proven software engineering principles. The relational paradigm, however, is based on proven mathematical principles. Because the underlying paradigms are different the two technologies do not work together seamlessly.
Of course to suggest that mathematics and software engineering are in such a dire conflict that they give birth to impedance mismatch, is utterly ridiculous. See ProgrammingIsMath.
The article is also littered with confusions and bad terminology (meaning not adhering to the consecrated terminology in databases, but rather one's own redefinitions). For example what is there considered "physical data model" is properly called "logical data model", or even simpler "logical schema", or just schema.
[General issues, with all host languages...]
See RelationalHostLanguageImpedanceMismatch, ObjectRelationalPsychologicalMismatch, TablesAndObjectsAreTooDifferent, ObjectRelationalImpedanceMismatch, TrueRelationalToPseudoRelationalImpedanceMismatch