OK, I'll bite ... It's been used in the context of Java RMI since
1996-7, DCOM since whenever that got going (1992?), in the context of
CORBA since whenever *that* got going (1987-8?), in the context of other
IDLs and RPCs (e.g. Sun RPC and XDR) since somewhere around 1983

... and IIRC in the context of compiler construction since about
Algol-60 when the basic theoretical foundations for almost everything
were laid ;-) In effect it's really part of any call-by-value system: a
data copy/formatting step into a format understood by the target.

"_why_ it is necessary"

but then how where people doing that when that buzzword didn't
exist?

You know, like if computing existed before that Microsoftism from
the DCOM era appeared (*).

;)

Seriously though, that page gives one reasonable definition of
marshalling but it doesn't say that the term *is* misused.

Does it imply validation as another poster said?

Does it imply going over the wire?  I'd think "yes", as does the
definition you gave.  But there are many people (blogs, articles,
Usenet postings) who definitely use the term for data that does
never leave the computer it was "marshalled" on.

Is it related to XML or not?

Is it related to XML in the Java world or not?

who coined the term? is it trademarked? if the answer is "nobody
knows" and "no" to these two questions, then it's not going to help
to have a precise definition (as opposed to, say, "Design by Contract"
which has been coined and trademarked by Meyer, this helps
to narrow the definition [I like this example]).

Note that even when terms have very precise meaning (eg "inner" or
"nested" in Java, as defined by the JLS) they're being misused.  So
when you start with a vague term like "marshalling" that apparently
had various meanings through the computing ages, you're unlikely
to find many people correctly using the term (which helps spread
misconceptions).

Based on these facts, I tend to avoid the term as much as possible
in documentation/talks/whatever.  There's usually always a less
ambiguous way to explain what's going on (I'd rather write a
few more words than have people not understanding or
misunderstanding what I'm writing).

As an example, the OP asked:

OP: "....:Is is possible for an EJB client to marshal an object
OP: of class java.lang.Class to an EJB?"
OP: what does "marshalling" means in the above question?

What has gone wrong for such a question to appear?

To me there's more to it than simply saying "here's a definition,
computing wouldn't exist without it"... (slightly exagerating ;)

HTH,

(*) I was already programming since many years when I first encoutered
the term (actually I probably already had forgotten COBOL when I
first read the term "(un)marshal(l)ing"  (**) and I attributed it
back in the days to a DCOM-specific term, but I was wrong as
another poster pointed out).

(**) I wrote "(un)marshal(l)ing" for the spelling with a single
"l" is quite often seen ("english english" vs "amercian spelling" ?)