The transient is the first few milliseconds: the click of the kick, the crack of the snare, the pick on the string. It is where a sound announces itself, and hearing leans on it heavily; clip the attack off a piano note and the remainder is surprisingly hard to name.
Producers manage transients constantly, sharpening them for punch through a mix or softening them to sit instruments back. Compressors live on this boundary, and their attack settings are decisions about transients.
In reactive visuals, transients are the events worth spending motion on. Sustained energy suits glow and drift; the transient deserves the cut, the spawn, the structural change, because that is what the body braces for.