Third-party tested
Testing done by an independent lab rather than the manufacturer itself.
Third-party tested means a supplement has been checked by an independent laboratory with no financial stake in the result — as opposed to relying only on the manufacturer's own in-house testing. It typically verifies that the product contains the ingredients and doses claimed and is below limits for contaminants such as heavy metals.
Programs like USP Verified and NSF Certified are formal versions of third-party testing. The label tells you the check happened; it does not by itself say the product is more effective.