A variable in code is a named placeholder for a value assigned by the computer when the program runs, with applications including environment variables and constants that can be used to make code more flexible and readable. Variables tend to be used more often than constants due to their flexibility, but constants are useful when values should not be changed once set, such as in mathematical calculations where precision is crucial. Effective variable naming is essential for readability and collaboration, with organizations developing style guides to promote consistent coding habits, including the use of descriptive names that convey the purpose of each variable. Code linters can also help identify inconsistencies with established style guidelines, making it easier to develop and maintain readable code.