A symbol that represents an idea or thought in non-verbal communication is…

Concept

Communication theory sorts symbols by the nature of the referent — the thing a symbol stands for. A concrete symbol stands for a tangible object and normally resembles it, points at it, or is the object itself, so its meaning can be read off its form. An abstract symbol stands for something intangible — an idea, a thought, a feeling, a relationship — and cannot resemble its referent, because the referent has no physical form; its meaning is therefore carried entirely by learned, shared cultural association. The classification follows what is being stood for, not the channel used, not how openly the sign is displayed, and not how well it works.

Application

The stem fixes the referent as "an idea or thought". An idea has no physical form, so no sign can resemble it, and the concrete half of the pair is ruled out by definition. The intangible-referent half of the standard pair is named the abstract symbol, so that is the term the stem is asking for. Familiar non-verbal instances behave exactly this way: a heart shape stands for affection, a lit bulb for a sudden insight, the ∞ figure for endlessness. None of them looks like what it means; each works only because a community has learned the association.

Cross-check — what the other labels on offer would classify

• "Hidden symbol" would classify a sign by how openly it is displayed — a question of visibility to the audience, which leaves the nature of the referent untouched. A sign can be concealed and still stand for a tangible object.

• "Effective symbol" would classify a sign by the response it obtains — a question of outcome in the receiver. The same referent can be signalled effectively or ineffectively, so this axis says nothing about what is being represented.

• "Ideational symbol" points at ideation, the mental process of forming content, rather than at a category of sign; communication theory names the referent-type pair concrete versus abstract, and "ideational" is not a member of it.

Result: a symbol standing for an idea or thought is an abstract symbol.