n 1  :  a substance that in very small amounts is able to increase the activity of a catalyst  2  :  a binding site in a DNA chain at which RNA polymerase binds to initiate transcription of messenger RNA by one or more nearby structural genes  3  :  a chemical believed to promote carcinogenicity or mutagenicity .
 
 
 
Similar sounding terms:  hy·per·me·tria  pe·rim·e·ter  pe·rim·e·try   premature 
 
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