1. (

• plug-in ,
• add-on

) a software module that extends or enhances the capabilities of an existing application ;

2. (

• addition ,
• add-on ,
• improver

) a component that is added to something to improve it; "the addition of a bathroom was a major improvement"; "the addition of cinnamon improved the flavor" ;

3. (

• accessory ,
• appurtenance ,
• supplement ,
• add-on ,

) a supplementary component that improves capability ;

1. (

• add_on

) make an addition; "Let's add on to this" ;

2. (

• append ,
• add_on ,
• supplement ,
• affix ,

) add to the very end; "He appended a glossary to his novel where he used an invented language" ;