Medical Definition of tertiary
1: of third rank, importance, or value
2a: involving or resulting from the substitution of three atoms or groups <a tertiary salt>b: being or containing a carbon atom having bonds to three other carbon atoms <an acid containing a tertiary carbon>c: of, relating to, or being the normal folded structure of the coiled chain of a protein or of a DNA or RNA—compare primary 4, secondary 3
3: occurring in or being a third stage <tertiary lesions of syphilis>
4: providing tertiary care <a tertiary medical center>
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