To add to the other answers: there is a historical reason for this. I can write this myself, but quoting Wikipedia is easier on the fingers:
JavaScript was originally developed by
Brendan Eich of Netscape under the
name Mocha, which was later renamed to
LiveScript, and finally to JavaScript.
LiveScript was the official name for
the language when it first shipped in
beta releases of Netscape Navigator
2.0 in September 1995, but it was renamed JavaScript in a joint
announcement with Sun Microsystems on
December 4, 1995 when it was deployed
in the Netscape browser version 2.0B3.
[…]
JavaScript very quickly gained
widespread success as a client-side
scripting language for web pages. As a
consequence, Microsoft developed a
compatible dialect of the language,
naming it JScript to avoid trademark
issues. JScript added new date methods
to fix the non-Y2K-friendly methods in
JavaScript, which were based on
java.util.Date. JScript was included
in Internet Explorer 3.0, released in
August 1996. The dialects are
perceived to be so similar that the
terms "JavaScript" and "JScript" are
often used interchangeably. Microsoft,
however, notes dozens of ways in which
JScript is not ECMA-compliant.
In November, 1996 Netscape announced
that it had submitted JavaScript to
Ecma International for consideration
as an industry standard, and
subsequent work resulted in the
standardized version named ECMAScript.
As you can see, the standard, ECMAScript, was developed later than the original language. It's just a matter of adapting this standard in the current implementations of web browsers, that's still going on, as is the development of ECMAScript itself (e.g., see the specification of ECMAScript 5, published December 2009).