Bioavailability appears on a great many supplement labels and is defined on almost none of them. It is a real pharmacological term with a specific meaning, and knowing that meaning turns a vague marketing word into a question you can actually ask.
The definition
Bioavailability is the proportion of an administered dose that reaches systemic circulation in a form the body can use. Swallow 100 mg of something, and if 20 mg ends up circulating unchanged, bioavailability is twenty percent.
By definition an intravenous dose is one hundred percent — it goes straight into the bloodstream. Everything swallowed is measured against that benchmark, and for plant compounds the number is often far lower than people expect.
The three places a dose gets lost
Dissolution. A compound has to dissolve before it can be absorbed. Fat-soluble compounds in a watery intestine dissolve poorly without dietary fat present to trigger bile release.
Transit across the gut wall. Some molecules cross easily; others are too large, too charged, or are actively pumped back out by transporters in the intestinal lining.
First-pass metabolism. Everything absorbed goes to the liver before general circulation, and the liver modifies a share of it immediately. For curcumin this stage alone removes most of what made it through the first two.
What "enhanced bioavailability" usually means
Several approaches exist and they are not equivalent. Piperine from black pepper slows the metabolism of certain compounds so more survives. Phospholipid complexes package a compound with fats so it behaves as though it arrived with a meal. Particle-size reduction increases surface area to improve dissolution. Liposomal preparations wrap compounds in lipid spheres.
Each has evidence behind it of varying quality, and the term on its own guarantees nothing. "Enhanced bioavailability" with no named method is a claim, not information.
The question it should make you ask
Not "is this bioavailable?" — almost anything is, to some degree. The useful question is narrower: what has been done about the ingredients that absorb badly, and what am I being asked to do?
For a formula containing lutein and turmeric, the honest answer often involves you. Taking the capsule with a meal containing fat is an absorption intervention, and it is free. A label that simply says "take with food" without explaining why has given you the instruction and withheld the reason.
Where it stops mattering
Bioavailability is necessary and not sufficient. A compound can be beautifully absorbed and still do nothing useful, because absorption says only that it got into circulation — not that it acts on anything once there.
That is why absorption is a chapter rather than the book. It explains why a reasonable product can underdeliver. It does not, on its own, make any product work.