Tetra hop / iso-extract bittering also: Tetrahydro-iso-alpha-acids, Iso-extract, Pre-isomerized hop extract
The original commercial hop extract. Industrial bittering since the 70s, light-stable and skunk-proof.
What it is, in plain English
Pre-isomerized hop extracts have been used by commercial brewers since the 1970s, primarily for bittering. They contain alpha acids that have already been chemically isomerized into iso-alpha acids (the bitter compounds you'd normally produce by boiling hops). Tetrahydro-iso-alpha-acids (Tetra) go a step further — they're chemically reduced so they can't react with light to produce 'skunked' beer. That's why most pale lagers in clear or green bottles use Tetra: the beer can sit on a shelf in sunlight without going off.
The technical version
Hops are extracted with CO2 or ethanol, the alpha acids are isolated, then chemically isomerized (and optionally reduced/hydrogenated) to produce a stable, water-soluble bittering compound. The reduced 'tetra' version is light-stable. Standard iso-extract is photosensitive like normal hops.
How brewers use it
Added cold-side, post-fermentation. Allows precise IBU adjustment without hop dosing. Used in clear-bottle and green-bottle lagers (Heineken-style) to prevent light-struck off-flavor. Industrial-scale only — too expensive and unfamiliar for most craft brewers. Some craft producers use small amounts of iso-extract as a 'trim' to fine-tune bitterness post-fermentation in delicate beers.
Who uses it
Macro lager producers globally. Almost no craft adoption. The product line predates modern hop-aroma extracts by decades.