a phrase describing a phenomenon where changes to one element are likely to have rippling or unpredictable effects on others, usually in the context of an analytical model with nested or interlocking parts, as in “We could change the market share assumption, but that’s going to have an accordion effect on the rest of the numbers”; derives from the movements of an accordion while it is being played, where the bellows expands and contracts dynamically in response to squeezing movements from both halves of the instrument; the term originates in physics and engineering, where a common use is to describe the bunching and slowing of groups of vehicles as they proceed down a road