addition ; [countable] a thing that is added to something else. All of these technologies are fairly recent additions. ; [uncountable] the act of adding something ...

used when you want to mention another person or thing after something else In addition to these arrangements, extra ambulances will be on duty until midnight.

There are 12 meanings listed in OED's entry for the noun addition. See 'Meaning & use' for definitions, usage, and quotation evidence.

a chemical reaction in which the whole of one molecule combines with the whole of another to form a single larger molecule

ADDITION definition: 1. the process of adding numbers or amounts together: 2. as well (as): 3. something that has been…. Learn more.

[transitive] to put something together with something else so as to increase the size, number, amount, etc. add something Next add the flour.

​more than was first mentioned or is usual synonym extra. additional resources/funds/security/funding/costs; The government provided an additional £25 ...

There are two meanings listed in OED's entry for the word addition–elimination. See 'Meaning & use' for definitions, usage, and quotation evidence.

​in a way that is more than was first mentioned or is usual synonym in addition (to somebody/something). Additionally, the bus service will run on Sundays, ...

​a thing that is added to something else. The company offers scuba-diving as an add-on to the basic holiday price. add-on software (= added to a computer).