Real Dictionary

add
ADD is an abbreviation for attention deficit disorder.
verb: If you add numbers or amounts together, you calculate their total.
verb: If you add one thing to another, you put it in or on the other thing, to increase, complete, or improve it.

ADD in American English
American Dialect Dictionary
abbreviation: attention deficit disorder
intransitive verb: to perform the arithmetic operation of addition
transitive verb: to unite or join so as to increase the number, quantity, size, or importance

add in American English 1
to join or unite (something) to something else so as to increase the quantity, number, size, etc. or change the total effect

add in American English 2
let there be added; add

add in British English
abbreviation for: attention deficit disorder
noun: an instance of adding someone to one's list of contacts on some social networking sites
verb: to combine (two or more numbers or quantities) by addition

attention deficit disorder in British English
noun: a disorder, beginning in childhood, characterized by excessive activity and inability to concentrate on one task for any length of time