Diacaustic is a relatively obscure word that refers to the use of double mirrors, also known as parabolic mirrors, to focus sunlight into a concentrated beam. However, there are several synonyms that can be used depending on the context in which the term is being used. Synonyms for diacaustic include "solar lighting," "solar focusing," "solar energy concentration," "mirrored light," and "parabolic concentration." These words are often used interchangeably to describe the same technology or process. While diacaustic may not be a commonly used term, its synonyms are more commonplace, and they are often used in discussions about renewable energy, lighting design, and other related fields.