Alpha acid is the class of resin compounds in hops responsible for bitterness once isomerized by boiling — measured as a percentage of a hop's total weight, and printed on virtually every hop package as the single most important number for predicting how bitter a given addition will taste.
Why the same weight of two hop varieties can taste completely different
A hop's alpha acid percentage is the single biggest driver of how much IBU a fixed weight of it contributes at a given boil time and gravity — a low-alpha noble variety at 3-5% alpha needs several times the weight of a high-alpha American variety at 12-15% alpha to hit the same measured bitterness, which is exactly why English bitter-family styles on this site call for so much more hop mass by weight than an American IPA needs for a comparable BU:GU ratio.
Alpha acid isn't fixed — it varies by crop year
A hop variety's alpha acid isn't a single constant number the way it's sometimes treated in casual recipe writing; it varies meaningfully between growing seasons and even between harvest lots of the same variety, which is why fresh hop packaging lists a specific tested percentage rather than a generic textbook figure. Running the actual printed alpha percentage through the IBU calculator, rather than a remembered rough number, is the difference between a recipe landing close to target and missing it by a meaningful margin.