A Word is a theme in the Symphony. The most famous Words are those of the Archangels and Demon Princes, but there are many, many others. Everyone in Heaven knows Flowers, the Word of the Archangel Novalis, but there's also the non-Superior Angel of Roses working beneath her.
A Word can include metaphorical implications as well. For instance, in addition to literal flowers, Novalis' Word encompasses things that flowers symbolize, such as peace and flourishing. Words do have independent existence as part of the Symphony -- the Word of Fire existed within the stars long before the first humans -- but as the Word of Flowers illustrates, human thoughts (being part of the Symphony) can also have an effect.
Words are not always either inherently infernal or divine. For example, Technology could easily be a Heavenly Word: but it is currently held by Hell. The more positive sides of technology were once covered by the Word of Knowledge, before the Archangel Raphael's destruction, and have been adopted (to the best of his ability) by Jean, Archangel of Lightning.
This leads to another point: Words can be shared but rarely are, especially not at the Superior level. A force known as "Word friction" has deleterious effects on the health and sanity of celestials sharing a Word with someone on the other side. If the power difference is significant, the weaker celestial is often simply annihilated: if Lucifer were to hand the Word of Stone to any but the most extraordinarily powerful demons, they would be burned to cinders by the Word friction with David. The only example of Superiors sharing a Word are Belial and Gabriel. Belial has been driven stark raving mad by the Word friction, and Gabriel is faring only somewhat better than he is these days.
Lastly, becoming Word-bound is a very serious matter. A celestial bound to a Word in essence becomes that Word. It is extremely hard (and painful) to unbind a celestial from their Word. If a Word fades out, the celestial goes with it. In such a case, most celestials are at least as upset about the Word's disappearance as their own, as again the distinction is quite blurry. Partially as a result, Heaven is reluctant to tie angels to smaller Words. Hell, on the other hand, frequently sticks demons with minor Words Sometimes this is a punishment -- ask the Demon of Smelly Size 3 Bowling Shoes -- but just as often it is the goal of the demon in question. Celestials' greater propensity (relative to humans) to fixate on things aside, the main explanation is probably that, while tying oneself to a minor Word risks a slow and painful demise, there are a lot of ways to die miserably in Hell, and a Word is often on balance a life-extender. Or at least an instant promotion that lets the demon get vengeance on all those jerks who've wronged them in the past...