Chelate / chelated mineral
A mineral bound to an organic molecule (often an amino acid, like glycine) rather than an inorganic salt (like oxide or carbonate). Chelation is marketed as improving absorption, and some chelated forms genuinely do outperform their inorganic counterparts in trials -- but "chelated" isn't a single defined compound, and quality varies by which chelating agent is used and how.
Example
Magnesium glycinate (a glycine chelate) is Tier 1 on our scale; "magnesium amino acid chelate," a vaguer label declaration, is Tier 2 -- lower urinary/salivary response than citrate in the same trial, since "amino acid chelate" isn't one specific compound.