The official definition for bioavailability is:

“the proportion of a drug or other substance which enters the circulation when introduced into the body and so is able to have an active effect”

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Put simply, if you take 10mg of something and your body can only effectively absorb 5mg, that would be 50% bioavailable.

You can read more about bioavailability  here - https://cbd-one.co.uk/common-questions