While reserved words are False, None, True, and, as, assert, async, await, break, class, continue, def, yield, elif, else, except, finally, for, from, global, if, import, in, is, lambda, nonlocal, not, or.

In Python, defining variable identifiers requires following rules:

1. Variable names can only consist of letters, numbers and the underscore "_", and cannot start with a number.

2. Variable names are case-sensitive, that is, variable names apple, APPLE and Apple represent three different variables.

3. You can't reserve words in Python, such as if, def, for, etc.

4. Variable names should be as meaningful as possible to facilitate the understanding and maintenance of the code.

5. In order to enhance readability, underscores should be used to separate long variable names.

6. Variable names should avoid using single lowercase letters L, uppercase letters O and I, which are easily confused with the numbers 1 and 0.

Following these rules can avoid introducing unnecessary errors and confusion in Python programs and improve the readability and maintainability of variables.