Laurence Sherwood asks:

>("ad libitum", which probably is not Italian).

It's Latin.

>Could someone describe how the two terms (rubato, ad libitum) differ?

It is my understanding that rubato applies solely to fluid, highly
localized variations in tempo.  Ad libitum applies more generally to
everything; indeed, in its colloquial usage, it means made up or improvised.

>Are they in any sense interchangeable?

I don't believe so.  Rubato has a very specific meaning.

len.