Millennium was released by John I. Haas around 2000 — its name a nod to the turn of the millennium rather than any technical detail — descended from Nugget breeding stock and developed specifically to push Nugget's dependable, moderately clean bittering character to a considerably higher alpha acid, matching the demand for more efficient bittering hops that was reshaping American IPA through the 2000s.
At 14.5-16.5% alpha acid, Millennium delivers strong bittering efficiency from a modest weight, and it's notable among high-alpha hops for an unusually clean bitterness relative to its alpha level — its cohumulone content runs lower than several of its high-alpha peers like Apollo or Zeus, which means the sharpness typically associated with very high alpha acid is comparatively muted here. Its aroma is generally described simply as mild herbal, present but not distinctive enough to define a recipe on its own. Millennium's alpha is efficient and comparatively smooth, which the IBU calculator will translate into an actual figure for a Millennium-based charge. Nugget is the direct genetic relative and closest substitute, offering a similar clean character at meaningfully lower alpha acid, requiring a heavier weight to match Millennium's bittering output. Magnum is the second substitute for a comparably neutral, efficient bittering profile. American IPA and double IPA use Millennium almost entirely as an early-boil bittering charge rather than a late-addition or dry-hop variety.