What does "RDA" mean?

RDA:

RDA [or Responsive Display Ad] is a type of advertisement at Google that adjusts its size and content based on the size of the space available for display, much as a "responsive" image code will display an image in the space available to it on a screen. An RDA may display as a big display ad on one web page, but as a text ad on a different web page where it has less space to display. RDA is the default ad type at Google.

 Google: How to Make an RDA