critical quality attribute
Of all the things you could measure about a product, only some really decide whether it is good. A coffee can vary in cup colour, foam and label print, but what truly matters is its temperature and strength. A critical quality attribute is one of those make-or-break properties of a medicine: a physical, chemical, biological or microbiological characteristic that must stay within an appropriate limit, range or distribution to ensure the desired product quality.
Formally defined in ICH Q8, a CQA is a property of the drug substance, an excipient, an intermediate or the final product that affects the product's safety or efficacy. Typical CQAs for a tablet include assay (how much drug), content uniformity, dissolution, related substances (degradation impurities) and, for sterile products, sterility and endotoxin level. The whole point of identifying CQAs is to focus development and control on the few attributes that actually matter to the patient.
Criticality is a matter of degree judged by risk, not a fixed label, and it can be debated. An attribute is critical if varying it within plausible bounds could harm safety or efficacy; one that has no patient impact across its realistic range is not critical even if it is measured. The honest difficulty is that judging criticality requires real understanding of the drug, and getting it wrong — overlooking a true CQA — is exactly how quality problems slip through.
For an immediate-release tablet, dissolution is a critical quality attribute: if too slow, the drug may not be absorbed in time, so a specification such as "not less than 80 % dissolved in 30 minutes" guards it.
A CQA is enforced through a specification with a defined acceptance limit.
CQAs (what the product must be) are distinct from critical process parameters (what you must control to get there); QbD work is largely about linking the two by cause and effect.