A variable in coding is a named placeholder for a value that the computer assigns when the program is run, allowing flexibility and reusability in coding. Environment variables are variables whose values are set outside of the program context, often used for security or implementation-specific reasons. Constants, on the other hand, can only be assigned once and are useful for situations where a value should not be changed. Naming conventions are crucial for readability and maintainability, with organizations developing style guides to ensure consistency in variable naming across projects and teams. Coding tools like linters can help enforce these guidelines and promote consistent coding habits.