additive
US /ˈæd.ə.t̬ɪv/
UK /ˈæd.ə.t̬ɪv/

1.
a substance added to something in small quantities to improve or preserve it
:
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Many processed foods contain artificial additives.
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This fuel has special additives to improve engine performance.
1.
relating to or denoting a substance added to something to improve or preserve it
:
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The company specializes in additive manufacturing processes.
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They are developing new additive technologies for plastics.