The phrase "is valid" implies that something is legitimate, true or accurate. Some synonyms for this phrase include "is genuine", "is authentic", "is legitimate", "is accurate", "is lawful", "is reliable", "is credible", "is potent", "is truthful" and "is sound". These words can be used interchangeably depending on the context and the level of formality required. For instance, "is sound" may be more appropriate in a scientific or technical context, while "is genuine" may be more suited to a social or relational setting. Overall, using synonyms can diversify your language and communicate more precisely to your audience.