What is the difference between JavaScript and ECMAScript?

ECMAScript is the language, whereas JavaScript, JScript, and even ActionScript 3 are called "dialects". Wikipedia sheds some light on this.

Technically ECMAScript is the language that everyone is using and implementing -- it is the specification created many years ago when Netscape and Microsoft sat down and attempted to standardise the scripting between JavaScript (Netscape's scripting language) and JScript (Microsoft's).

Subsequently all these engines are ostensibly implementing ECMAScript, however JavaScript (the name) now hangs around for both traditional naming reasons, and as a marketing term by Mozilla for their various non-standard extensions (which they want to be able to actually "version")

Various JavaScript versions are implementations of the ECMAScript standard.

JavaScript is a ECMAScript language.

ECMAScript isn't necessarily JavaScript.

Here are my findings:

JavaScript: The Definitive Guide, written by David Flanagan provides a very concise explanation:

JavaScript was created at Netscape in the early days of the Web, and technically, "JavaScript" is a trademark licensed from Sun Microsystems (now Oracle) used to describe Netscape's (now Mozilla's) implementation of the language. Netscape submitted the language for standardization to ECMA and because of trademark issues, the standardized version of the language was stuck with the awkward name "ECMAScript". For the same trademark reasons, Microsoft's version of the language is formally known as "JScript". In practice, just about everyone calls the language JavaScript.

A blog post by Microsoft seems to agree with what Flanagan explains by saying..

ECMAScript is the official name for the JavaScript language we all know and love.

.. which makes me think all occurrences of JavaScript in this reference post (by Microsoft again) must be replaced by ECMASCript. They actually seem to be careful with using ECMAScript only in this, more recent and more technical documentation page.

w3schools.com seems to agree with the definitions above:

JavaScript was invented by Brendan Eich in 1995, and became an ECMA standard in 1997. ECMA-262 is the official name of the standard. ECMAScript is the official name of the language.

The key here is: the official name of the language.

If you check Mozilla 's JavaScript version pages, you will encounter the following statement:

Deprecated. The explicit versioning and opt-in of language features was Mozilla-specific and are in process of being removed. Firefox 4 was the last version which referred to a JavaScript version (1.8.5). With new ECMA standards, JavaScript language features are now often mentioned with their initial definition in ECMA-262 Editions such as ECMAScript 2015.

and when you see the recent release notes, you will always see reference to ECMAScript standards, such as:

• The ES2015 Symbol.toStringTag property has been implemented (bug 1114580).

• The ES2015 TypedArray.prototype.toString() and TypedArray.prototype.toLocaleString() methods have been implemented (bug 1121938).

Mozilla Web Docs also has a page that explains the difference between ECMAScript and JavaScript:

However, the umbrella term "JavaScript" as understood in a web browser context contains several very different elements. One of them is the core language (ECMAScript), another is the collection of the Web APIs, including the DOM (Document Object Model).

Conclusion

To my understanding, people use the word JavaScript somewhat liberally to refer to the core ECMAScript specification.

I would say, all the modern JavaScript implementations (or JavaScript Engines) are in fact ECMAScript implementations. Check the definition of the V8 Engine from Google, for example:

V8 is Google’s open source high-performance JavaScript engine, written in C++ and used in Google Chrome, the open source browser from Google, and in Node.js, among others. It implements ECMAScript as specified in ECMA-262.

They seem to use the word JavaScript and ECMAScript interchangeably, and I would say it is actually an ECMAScript engine?

So most JavaScript Engines are actually implementing the ECMAScript standard, but instead of calling them ECMAScript engines, they call themselves JavaScript Engines. This answer also supports the way I see the situation.

Javascript was the original name, meant to capitalize on the popularity of java. ECMA is the standards body that oversees the standard that was eventually put in place so the names are roughly equivalent.

Implementations have other names, like V8 or Rhino, etc.

I doubt we'd ever use the word "ECMAScript" if not for the fact that the name "JavaScript" is owned by Sun. For all intents and purposes, the language is JavaScript. You don't go to the bookstore looking for ECMAScript books, do you?

It's a bit too simple to say that "JavaScript" is the implementation. JScript is Microsoft's implementation.

JavaScript is one branch of languages formed around the ECMAScript standard. I believe ECMA is the European Computer Manufacturers Association, not that this is really relevant or anything.

Don't forget another popular language formed around the ECMA Script standard is ActionScript, used in Adobe Flash/Flex.