The definition is a bit subjective, but not very much: A programming language is compact, if the program size is small for the typical programs we want to write. (The subjective part is what we call a typical program.)

Currently I think that the programming language Joy is the best candidate to be the base for creating the most compact programming language in the world.

http://en.wikipedia.org/wiki/Joy_(programming_language)

I have quickly defined a variant of Joy, and I have defined an efficient coding scheme for it. (This is still on paper, I did not write it down precisely enough to publish). It turned out that the really necessary tokens of joy can be typically encoded in 3-4 or in the worst case in 5 bits. This means that the quicksort algorithm mentioned in the wikipedia articel is approximatley 9 bytes long.

Do you have an idea for an even more compact programming language? Do you think that a cleverly encoded Joy-variant is near some theoretical limit of compactness?