Following are the best practices while declaring a variable.
Variables names should be short or long enough as per the scope. For example, loop counter variable, i is fine whereas employee as a loop variable.
Specific words should not be used as equals, compare, data.
Use meaningful names which can explain the purpose of the variable. For example cnt Vs counter.
Don't use _ to declare a variable name, Use camel casing. For example, employeeName is better than employee_name.
Each organization has its own syntax specific standards. Follow those rules to maintain consistency and readability.