A term coined by John Starrett in a Tuning List posting. It refers in his case specifically to the use of certain intervals avalable in 19-edo which aurally resemble intervals familiar from 12-tET in their melodic progression, but which, because of their different size from 12-edo, modulate harmonically into unfamiliar tonal regions that lie 'between the cracks' of the 12-edo pitches.
Use of the term can be generalized to apply to any microtonal tuning system, the main point being the 'punning' involved in the reference to familiar intonational paradigms and their skewed realization in the less-familiar actual tuning.
The tonalsoft.com website is almost entirely the work of one person: me, Joe Monzo. Please reward me for my knowledge and effort by choosing your preferred level of financial support. Thank you.