The term 'in theory only' can be expressed in two ways: 'THEORETICAL' and 'onpaper'. The word 'THEORETICAL' is an 11-letter word that refers to something that exists or is done in theory, but not in practice. On the other hand, 'onpaper' is a 7-letter word that means something that is written down on paper, but not necessarily implemented in real life. In other words, both terms refer to something that is only conceptualized and not actually put into practice.