In a strictly scientific sense, the term "bioactive" is an alternative term for "biologically active". A bioactive compound is therefore simply a substance that has a biological activity, like activators, inhibitors, effectors etc.
Compound libraries, selections of varying numbers of small molecules or bioactive compounds dedicated to a certain topic, pathway or context, are widely used in target identification, high-throughput or focused screenings for drug, drug repositioning / repurposing or new target discovery.