The word solid refers to something that is firm and not easily broken or dissolved. There are plenty of synonyms that are associated with the word solid. Some of them include sturdy, durable, stable, strong, tough, firm, and steadfast. The usage of any of these synonyms would be appropriate based on the context in which you are using the word. Other similar synonyms for the word solid include substantial, weighty, convincing, reliable, and concrete. These words can be used interchangeably to convey the same idea or thought in different contexts and situations. Using the right synonym can add value and depth to your writing.