If you say, "George, this is Joan" have you just introduced George to Joan or have you just introduced Joan to George? I've seen this sort of thing many times and it has never been clear who is the introduced and who is the introduced to.
EDIT: All of the answers so far are too complicated. I just need a simple answer. Have you just introduced George to Joan or have you just introduced Joan to George?
If you say, "George, this is Joan", have you just introduced George to Joan or have you just introduced Joan to George?
You have introduced Joan to George.
See definition 2 in Oxford dictionary.
Introduce in this context means "make known". The direct object is the person you make known (Joan). The indirect object - which is optional - is the person or people whom you are addressing.
Other examples of introducing Joan:
Class, this is Joan, a new student. (introducing Joan to the class)
Attention everyone! I'd like to introduce Joan, your new teacher. (Note: no "to" in the sentence).
You introduce someone else to the person you are talking to. You addressed George, so you are introducing Joan to him. Other actions besides introduction follow this pattern. If I say "Dad, this is my new book", I am showing (or giving) my new book (second item named) to my father (the person initially addressed).
If you then wished to introduce George to Joan, you would then say "Joan, this is George". In formal etiquette (certainly in Britain, I don't know about elsewhere) there are rules about who goes first: men are introduced to women; junior people are introduced to seniors (whether by age or rank etc); a new arrival is introduced to a group.
In an informal setting, people might say "Joan, meet George, George, meet Joan" or even just "Joan, George, George, Joan.", and they might ignore the junior/senior men/women priority rules.
Breaking it down, when we say:
George, this is Joan
We are addressing George, but we are introducing George and Joan to each other.
From Oxford:
introduce (verb) make (someone) known by name to another in person, especially formally:
I hope to introduce Jenny to them very soon.
Unless one of the two parties is behind a pane of one-way glass, or George and Joan are in a room where music is blaring so loud that only one person can barely make out what their friend is saying, as soon as one says, "George, this is Joan," then George becomes aware of Joan's name, and Joan also becomes aware of George's name.
Sometimes, for the sake of politeness, one might "complete" the introduction by saying the names in the opposite direction:
George, this is Joan; Joan, this is George.
Or, more tersely:
George, this is Joan. Joan, George.
but some might find that second part redundant because no information is given. Still, the longer version might seem more polite; otherwise, Joan might feel left out of the introduction.
Asking which person is introduced to whom is sort of like asking, during conception, does the sperm fertilize the egg, or does the egg get fertilized by the sperm? The introduction happens, and George and Joan both learn each other's names.
If, on the other hand, you only said to George, "This is Joan," then George has been introduced to Joan, but Joan hasn't been introduced to George yet.
