Full Idea
In chemistry the 'thick' notion of a mechanism traces out positions of electrons and atomic cores, and correlates them with energies, showing the whole reaction. 'Thin' mechanisms focus on a discrete set of intermediate steps.
Gist of Idea
Thick mechanisms map whole reactions, and thin mechanism chart the steps
Source
Weisberg/Needham/Hendry (Philosophy of Chemistry [2011], 5.1)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.30