Neoplatonic symbolism is closely related to the doctrine of theurgy, which was introduced by Iamblichus of Chalcis. The language of Iamblichus' writings is distinguished by a variety of technical terms that require clarification. Among them are symbolon and synthēma, which are treated as fully equal by some scholars, but are clearly discerned by others. The goal of the present study is to define the meaning of symbolon and synthēma in the extant writings of Iamblichus using context analysis and relevant dependencies on the works of Iamblichus' predecessors, especially Porphyry of Tyre. Four major subject areas were selected: divine names and hieratic formulae, which are mostly called synthēmata; general theoretical descriptions of symbolic expression of intellectual entities in the material world; Pythagorean acousmata, which are traditionally called symbols; and numeric symbols, also interpreted in a Pythagorean manner. Iamblichus ascribes synthēmata self-sustained nature and the possibility to act on their own, revealing their theurgic efficacy on the human soul and intellect. Symbols are considered an explication of synthēma in a particular object, they have various forms and reveal the corresponding synthēma in a concealed or enigmatic way, thus requiring an interpretation. The combination of symbol's significative and efficacious aspects distinguishes it from sign, image, and analogy.