When defining a variable, what must be considered for it to be valid?

For a variable to be valid, it is essential that it may start with a letter or an underscore. This rule is a common convention in many programming languages. A variable's name typically must begin with a letter (A-Z, a-z) or an underscore (_) followed by any combination of letters, digits (0-9), and underscores. This allows for flexibility in naming variables while avoiding conflicts with reserved keywords that may be used by the programming language.

In contrast, a variable containing only numeric digits or no characters at all would limit its effectiveness, as it would either not be recognized as a valid identifier (if it starts with a number) or be deemed of little use. Additionally, being a reserved keyword would invalidate the variable name, as reserved keywords are exclusively used by the programming language for specific operations and cannot be repurposed as variable names.