"Apostilb" is a scientific term that refers to the unit of luminance in the photometric system. Due to its specific nature and usage, there are not many synonyms available for this term in common language. However, some similar terms that are often used in related contexts include "candela per square meter" and "nit." These terms are often used in the measurement of light intensity and luminance, and they can be used as substitutes for "apostilb" in certain cases. Additionally, some technical terms such as "foot-lambert" and "lumen per square foot" can also be used as synonyms for "apostilb" in the field of lighting and optics.