A VJ loop is a few seconds of video engineered to cycle without a visible seam, usually beat-synced lengths (one, two, four, eight bars) so it locks to tempo when pitched. Libraries of them, sorted by energy and style, are the ammunition of clip-based sets.
A large marketplace exists around packs, and craft matters: clean loop points, headroom for effects, motion that reads at low bitrates in dark rooms, codecs like HAP that scrub instantly.
Loops trade freshness for reliability, which is the deal generative systems refuse: software like Photism renders each moment from the live audio instead, so nothing repeats, at the cost of the VJ’s shot-by-shot control.