the.com/additive

Something small added to change everything, legally or chemically, your pick.

means A substance mixed into something else in small amounts to change its properties — flavor, color, shelf life, stability — without becoming the main ingredient itself.

from Traces back to the Latin verb addere, 'to add' (ad- 'to' plus dare 'to give'), through additivus, meaning 'that which is added.' The adjective form existed first in English, describing anything with the quality of adding to something else; the noun sense — a substance put into food, fuel, or other materials — is a much later, more technical spin-off, riding in on 20th-century chemistry and industry.

food version
keeps bread soft for weeks, unsettlingly
math version
property where order of adding never matters
etymology
from latin addere, to add to
legal use
contracts add it as extra terms, not subtract
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