Chelate / chelated mineral
A mineral bound to a molecule at two or more points, forming a ring; in supplements the ligand is usually an amino acid such as glycine, as opposed to 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 rated Tier 1 on our scale, on the strength of limited evidence; "magnesium amino acid chelate," a vaguer label declaration, is Tier 2 -- lower serum and salivary magnesium than citrate in the same trial, and "amino acid chelate" isn't one specific compound.