There are quite a few kinds of theoretical terms as defined: Terms for abstract things, like "democracy" or "justice"; terms for impossible things, like "square circle" or "negative squared number"; terms for non-existent things, like "griffon" or "mermaid"; terms for possible things in the future, like "the next World War" or "the state of the world in 500 years"; terms for things that are not revealed even if they may be unrevealed like "the inside of this" or "the backside of that"; and terms for things in the past that are not known, like "what Jezus preached" or "the precise number of trees in London in 1700" etc.

The reason to define theoretical terms as I did is to stress that everybody relies on some theoretical terms in most of his reasoning, and that in fact most of the terms one uses are either theoretical or involve theoretical terms in their definition.

And the reason theoretical terms are important is that any theory that goes beyond the empirically known facts, and theorefore any theory that makes any prediction, involves ipso facto theoretical terms, even if these are very common sensical.

When speaking about scientific theories, it makes sense to use 'theoretical' in a modalized sense: Theoretical terms are such as cannot be experienced in suitable conditions with appropriate procedures. They may still be both wholly justified and completely unavoidable, but must then be tested and supported by their empirical consequences. This also means that what starts out as a theoretical term (such as "atom") may become an empirical term, and conversely ("phlogiston").