In Java, an identifier is a name given to a class, method, variable, package, or constant that is used to uniquely identify it in a program. Identifiers can be a sequence of letters, digits, underscores, and dollar signs, but they must follow specific rules to be considered valid. Here are some of the rules for defining a valid Java identifier:

• The first character must be a letter, underscore, or dollar sign.
• Subsequent characters can be letters, digits, underscores, or dollar signs.
• Identifiers are case-sensitive.
• Identifiers cannot be a reserved word or keyword in Java.
• Identifiers cannot contain whitespace or special characters like @ or #.

It is recommended to use descriptive names for identifiers to create understandable and maintainable code. For example, "minutesPerHour" is a more descriptive name than "m" for a variable that stores the number of minutes in an hour.

Some examples of valid identifiers in Java are "Test" (class name), "main" (method name), "args" (variable name), and "totalVolume" (descriptive name for a variable) .

It is important to follow the rules for defining Java identifiers, as violating them can result in a compile-time error.