Transient response, certainly, as has been pointed out by others, sonically perceived as a lack of smear/crispness, unforced clarity and liquidity. Usually, not least through quality all-horns, I find it linked to uninhibited dynamics as well (or: ease of "explosivity," felt as a startling experience), coupled to (at least as a partially responsible aspect) a pronounced degree of low volume level "ignition," whereby music comes alive quite easily at lower SPL's.
