Why was the name 'let' chosen for block-scoped variable declarations in JavaScript?

Let is a mathematical statement that was adopted by early programming languages like Scheme and Basic. Variables are considered low level entities not suitable for higher levels of abstraction, thus the desire of many language designers to introduce similar but more powerful concepts like in Clojure, F#, Scala, where let might mean a value, or a variable that can be assigned, but not changed, which in turn lets the compiler catch more programming errors and optimize code better.

JavaScript has had var from the beginning, so they just needed another keyword, and just borrowed from dozens of other languages that use let already as a traditional keyword as close to var as possible, although in JavaScript let creates block scope local variable instead.

I guess it follows mathematical tradition. In mathematics, it is often said "let x be arbitrary real number" or like that.

Adding to exebook's response, the mathematics usage of the keyword let also encapsulates well the scoping implications of let when used in Javascript/ES6. Specifically, just as the following ES6 code is not aware of the assignment in braces of toPrint when it prints out the value of 'Hello World',

let toPrint = 'Hello World.';
{
    let toPrint = 'Goodbye World.';
}
console.log(toPrint); // Prints 'Hello World'

let as used in formalized mathematics (especially the writing of proofs) indicates that the current instance of a variable exists only for the scope of that logical idea. In the following example, x immediately gains a new identity upon entering the new idea (usually these are concepts necessary to prove the main idea) and reverts immediately to the old x upon the conclusion of the sub-proof. Of course, just as in coding, this is considered somewhat confusing and so is usually avoided by choosing a different name for the other variable.

Let x be so and so...

  Proof stuff

 New Idea { Let x be something else ... prove something } Conclude New Idea

 Prove main idea with old x