An enhanced version of ANSI screen codes.
Back in the old days of text-based bulletin board systems (BBSs), most of them used ANSI codes to change text colour, do basic animations and of the like. However, these increased the amount of data to transfer quite significantly.
Avatar was an alternative set of codes that could be used to do the same thing. However, the codes themselves were shorter, and it also included some basic codes to handle run length encoding of character streams - so rather than having, say, 40 As in the text (ie 40 bytes), you'd have a short code which says "repeat this 40 times" followed by the "A".
There were many programs (such as TheDraw) which could save screens in both ANSI and Avatar format, and also utilities to convert between the two. Of course, you needed to use terminal software that would recognise the codes... ANSI was pretty well universal, Avatar less so. I believe that some BBSs could store copies of all your graphical files in both formats, deduce what your client supported and send the appropriate ones.