often used seldom defined terms: lvalue

An lvalue is a value that can be assigned to:

lvalue = rvalue;

It’s short for “left value” or “lefthand value” and it’s basically just the value on the left of the = sign, i.e. the value you assign something to.

As an example of what is not an lvalue (i.e rvalue only):

printf("Hello, world!\n") = 100; // WTF?

That code doesn’t work because printf() (a function that returns an int) cannot be an lvalue, only an rvalue.