It's not totally APL-specific terminology, although it's uncommon in other areas of computer science. You can find it (predating APL) in areas like mathematical logic, where a predicate over two terms is sometimes called a "dyadic predicate". I'm guessing Iverson was familiar with that usage and extended it to APL. I wouldn't read too much into it though. It's mostly just Latin vs. Greek roots (unary, binary, ternary are Latin; monadic, dyadic, triadic are Greek).