The standard definition would be "the ability to render small-scale volume gradations." What I actually listen for is how clearly a headphone/IEM can reproduce the S and the K in this track. With something with poor microdynamics, at least one of those will get drowned out due to unevenness in said gradations. The midrange and upper bass also influence microdynamics, though, so I use a separate piano track for those and focus on the initial attack.

Also, it's precisely because the track is so busy that it's an effective stress test for gear. Generic Jazz Track in Audiophile CD Compilation #26 is comparatively poor for testing sound quality because it's undemanding and sounds good on everything, being slow-paced, relatively simplistic, and midrange-centric.