Mediated reference theory
The mediated reference theory is a semantic theory that posits that words reference something in the external world, but are mediated by some other process. One of the paradigm cases of a mediated reference theory was formulated by mathematician and philosopher Gottlob Frege.
Frege saw that the semantics of words and expressions could be divided up into two elements: a meaning (or sense) which is a (usually definite) description(s) by which we come to know the reference of an object; and the reference, which is the actual thing being referred to. For example, "the morning star" and "the evening star" are two different senses (i.e. meanings or modes or presentation) which both refer to the object Venus. Both "the morning star" and "the evening star", for Frege, are abstract objects which exist in a third realm of thoughts, independent of the mind or the external physical world.
Moreover, sentences and names have different kinds of senses and referents. The sense of a sentence is a proposition, or state of affairs; the reference is a truth value -- "true" or "false". The sense of a proper name is a concept that describes some person; the referent of a proper name is the actual individual in the world.
[Russell thought that names were a sort of description in disguise, which seems to be a similar position to Frege's. However, some scholars (such as Gareth Evans) have questioned whether Frege did hold such a view.]
There are some exceptions to the mediated reference theory, however. Some names don't seem to point to things in the world as their referent. For example, if a person utters the statement "It is common knowledge that 'Mark Twain' was an author", they are not just talking about the man Mark Twain, but also talking about whether or not people know and recognize something about the expression "Mark Twain". In this case, the expression itself - the signs, the string of words - are the sense of the proposition, and not just the content of the expression. In today's language, these exceptions are called opaque contexts.