A certain is a vague term used to describe something or someone that is not explicitly named or identified. There are many synonyms for a certain that can help to clarify or specify the meaning of a sentence or phrase. Some alternative phrases could include "a specific," "an undeniable," "an unmistakable," "an individual," "a particular," "a distinct," "a unique," "an absolute," "a definite," or "a specific group of." These synonyms can be used interchangeably with a certain to provide more descriptive and precise language that enhances the overall clarity and impact of a statement.