A
provisional truth used to preserve
mental bandwidth in the
educational process. Often, this is done without acknowledging that it is not really the truth. Sometimes, this is because the instructor
doesn't know it's a lie. Sometimes, it
strategic omission, because
inquisitive minds don't want to accept any
weak approximations. With more advanced students, it becomes safe to say that this is a
partial truth or an
engineering approximation, that is appropriate for some contexts, but not others.
Some people, even if very advanced, are quite opposed to the instructional lie. This is probably why they are often delivered undisclaimored.