Add-in

a component, as a video card or chip, added to a computer to expand its capabilities.
a small software program that adds a feature to another application.
to unite or join so as to increase the number, quantity, size, or importance:
to add two cups of sugar; to add a postscript to her letter; to add insult to injury.
to find the sum of (often followed by up):
Add this column of figures. Add up the grocery bills.
to say or write further.
to include (usually followed by in):
Don’t forget to add in the tip.
to perform the arithmetic operation of :
children learning to add and subtract.
to be or serve as an (usually followed by to):
His illness added to the family’s troubles.
Journalism. copy added to a completed story.
add up to, to signify; indicate:
The evidence adds up to a case of murder.
add up,

to make the desired, expected, or correct total:
These figures don’t add up right.
to seem reasonable or consistent; be in harmony or accord:
Some aspects of the story didn’t add up.

verb
to combine (two or more numbers or quantities) by addition
(transitive) foll by to. to increase (a number or quantity) by another number or quantity using addition
(transitive) often foll by to. to join (something) to something else in order to increase the size, quantity, effect, or scope; unite (with): to add insult to injury
(intransitive) foll by to. to have an extra and increased effect (on): her illness added to his worries
(transitive) to say or write further
(transitive) foll by in. to include
noun
(informal) an instance of adding someone to one’s list of contacts on a social networking site, esp MySpace: Thanks for the add!
abbreviation
attention deficit disorder
v.

late 14c., “to join or unite (something to something else),” from Latin addere “add to, join, attach, place upon,” from ad- “to” (see ad-) + -dere comb. form meaning “to put, place,” from dare “to give” (see date (n.1)). Meaning “to do sums, do addition” also is from late 14c. Related: Added; adding. To add up “make sense” is from 1942.

ADD abbr.
attention deficit disorder
ADD
Abbreviation of attention deficit disorder
Administration on Developmental Disabilities
attention deficit disorder