At the same instant can be replaced with several synonyms depending on the context. For instance, it could be replaced with simultaneously, instantly, promptly, concurrently, contemporaneously, coincidentally, at once, or immediately. These synonyms convey the idea of something happening at the exact same moment or time frame. For example, you could say, "The two cars collided simultaneously," or "The bomb detonated instantly upon impact." In general, the choice of synonym depends on the writer's preference and the intended message. However, it is important to use the synonym that perfectly fits the context to avoid ambiguity or confusion for the readers.