A variable is a container that stores value or information. 

Syntax:

variable_Name [ CONSTANT ] datatype [ NOT NULL] [:= | DEFAULT Initial_Value | expression];

some of the components of the syntax are optional.

Variable_Name:  this is the name of the variable. While it can be anything (must be valid), it's advisable to use meaningful names. The variable 'student_name' is 
betther than just 's'. There are all valid variable names.

CONSTANT: this is optional and should be used if the variable value does not be changed in the future. 

datatype: this defines the datatype of the variable. It can be a NUMBER or character datatype for example.

NOT NULL: it is optional. If not null, then a value must assigned to the variable.

:=  this is the assignment operator in PL SQL

DEFAULT initial value: is the initial value assignment to the variable. It can be boolean, number, null or character.

Variable Naming Convention

Here are some rules to observe when naming variables in PL SQL:

• Must start with a number
• Can contain special characters like _, #, $
• Maximum length of 30 characters
• Not an Oracle reserved word.