Best way to think of NULL is "no value." Not "unknown value," as that implies that it is a value you just don't know what it is. Not "missing" value as that even the notion of a value being "missing" tells you something. NULL is no value. It's like a black hole, it consumes anything it touches. Any expression involving a NULL term becomes NULL.
And to further apply semantics to this just to be snide, the value is only "missing" if it could possibly ever be defined for that tuple. There are cases where you expect the value to be "missing", and it not being "missing" would be considered a data integrity issue. Fun.