In JavaScript, a variable is a "named storage" for data. It is a container for storing data that can be changed later on. Variables are used to store information, such as numbers, text strings, and other data. In JavaScript, variables can be declared using the keywords var, let, or const.

• var: This keyword was used in all JavaScript code from 1995 to 2015. It can be used to declare a variable without initializing it, and it can also be used to declare multiple variables in one line.
• let: This keyword was introduced in ES6 and is now the preferred way to declare variables. It can be used to declare a variable and initialize it with a value, and it can also be used to declare multiple variables in one line.
• const: This keyword is used to declare a constant variable, which is like a variable except that it cannot be reassigned a new value after it has been initialized.

All JavaScript variables must be identified with unique names, called identifiers. Identifiers can be short names or more descriptive names. Variable names are pretty flexible as long as they follow a few rules, such as starting with a letter, underscore, or dollar sign, and not using any of JavaScripts reserved keywords.

In summary, a variable in JavaScript is a container for storing data that can be changed later on. It can be declared using the keywords var, let, or const, and must be identified with a unique name called an identifier.