"Syntax" is a type for representing source code in Racket, which
is a wrapper around S-expression (see a recent blog post for
details). "Syntax value" and "syntax object" are all synonyms of
this, and ni the ancient days of the mzscheme language functions
that deal with syntax used syntax-value in the name. These days
we use just "syntax" more often, and for a plural form we use
"syntaxes". An "S-expression" is either a primitive piece of
data that can be typed in code (symbols, numbers, strings,
booleans, etc -- in Racket you could also include other types),
or a list of these things. An S-expression is therefore any
nested structure of lists made of these primitive types at the
fringe. Sometimes this includes vectors too (since they can be
typed in using the #(...) syntax) but more usually they're left
out. Finally, "datum" is another name for an S-expression,
sometimes when you want to refer to the fact that it's a piece
of data that has an input representation. You can see how R5RS
introduces it: <Datum>