Longman Dictionary of Contemporary English homepage

add-on

 noun
     
Related topics: Computers
add-on [countable]
1 something extra that is added to an existing plan, agreement, law etc
add-on to
We bought legal protection as an add-on to our home insurance policy.
2TD a piece of equipment that you connect to a computer to improve its performance:
an add-on circuit board
3 a product that is designed to be used with another product
Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.

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