More precisely, a
value in a
computer program that is guaranteed to have a named
location (in
memory or in a
register) by the
language definition. Originally the name "lvalue" came from it being something that made sense on the
LHS of an
assignment statement (and obviously said LHS must have some named location, or we cannot
assign to it, or cannot ever find the assigned value again).
However, the distinction is (somewhat) important. C and (to a much great extent) C++ have a concept of constant lvalue! Naturally, it never makes sense to have a constant on the LHS of an assignment statement. But a statement like const double pi = 4*atan(1.0); does create an lvalue! We can say &pi to get a pointer (of type const double *).
It seems like the operative definition of an lvalue in C and C++ is "anything to which the address-of operator & may be applied". Of course, this still doesn't deal with overloading operator&() in C++; it might be better to think of an lvalue there as anything that may be passed by reference.
By contrast, you cannot take the address even of as simple an expression as 2*pi*r. So it's not an lvalue. And, of course, there's no reason for it to have a named location.