Words for 'the molecule itself'
Several terms all point at the active molecule, but with different emphasis. The active pharmaceutical ingredient (API) is the part of a pill that actually does the work — everything else is filler and coating. A brand-new molecule never approved before is a new chemical entity (NCE); regulators treat NCEs with special care precisely because they're unfamiliar. And the molecule formally chosen to advance toward trials is the drug candidate. Same physical thing at different moments of its life — like 'recruit', 'soldier', and 'veteran' all naming one person.
One important non-drug cousin: a chemical probe. A probe is a molecule built not to treat anyone but to *ask a question* — to test, in the lab, what a target does. Probes follow many of the same rules as drugs but are tools for science, not medicines. Confusing a probe for a drug is a classic beginner slip.
'Works' is not the same as 'drug-like'
Here is the idea that separates a chemist from a hobbyist. A molecule can have strong bioactivity — high potency against the target — and still be useless as a medicine, because it can't be absorbed, or it's destroyed in the gut, or it also hits a dozen other proteins. Hitting *only* the intended target is selectivity, and it matters as much as potency.
The catch-all word for 'has the properties a real oral medicine needs' is druglikeness. The most famous rough guide to it is Lipinski's rule of five, a back-of-envelope checklist suggesting that orally available small molecules tend to stay within certain size and greasiness limits. It is a guideline, not a law — plenty of fine drugs break it — but it captures a real truth: a good drug is a *balance*, never a single number maximized.