Defining the relationship

What is it?
Defining the relationship means discussing with people whom you are romantically involved (or potentially involved) about what your relationship specifically is, and what you want in that relationship.  This primarily includes whether you want something short- or long-term, a casual fling, or a serious committed relationship.  But there are many types of relationships: platonic friendship, friends with romantic feelings (but nothing sexual), "friends with benefits," one-night stand, "few-night" stands, casual sex (which could be a fling that is clustered for a finite time, or on/off over long-term), serious/committed, and even non-sexual long-term partners.

You could also discuss things such as whether you are seeing (or can see) other people, how much space/time you want, what to expect in your relationship, what you want/don't want, how quickly you want to progress sexually, and boundaries.  If you both want a serious relationship, you should also discuss at some point about being mutually exclusive.

Why define the relationship?
This conversation is not easy but is important to have so there is mutual understanding about what to expect.  This is a sign of respect and prevents feelings from being hurt later if there were discrepant intentions.  It will also promote an honest and open relationship.

Who should I define the relationship with?
Have this discussion with everyone you are involved with romantically (or potentially involved with), even if it is on the first date.  You don't want to lead anyone on.  

When should I define the relationship?
It is beneficial to have this discussion early (especially the type of relationship you want - casual or serious).  I would recommend talking about it within the first couple of dates, especially before you get intimate.  You don't want to waste time pursuing a relationship that will inevitably end because you both had different agendas.  You could also feel hurt/used if you became intimate, thinking you both were in a serious relationship, only to find out later that the other person wanted something casual. 

Even if you are both seeking a serious relationship, it doesn't mean that you are mutually exclusive.  You and/or your partner may be dating other people, and haven't decided which one to become committed to yet.  Note that dating more than one person does not necessarily translate into your partner not liking you, not taking your relationship seriously, or not being able to commit to a serious long-term relationship later.

Do not assume that you are mutually exclusive after going out on a certain number of dates or after a certain number of week/months, sleeping together, or hitting a particular milestone in the relationship (such as meeting their friends/family).  It is not official until it has been explicitly discussed and agreed upon.  So when does one have this discussion?  I personally think it's best to discuss being mutually exclusive once you have decided the other person is a potential good long-term partner.

Where should I have the discussion?
Discuss this personal and important matter when you are both not rushed for time, and in a private place in case emotions run high.  The exception may be someone you are meeting for the first time.  In that case, you will most likely discuss it in a public setting since that is where you are meeting.

How should I discuss it?
Explain that you want to have this discussion so you are both on the same page and know what to expect.  You don't want to waste anyone's time (yours or theirs), nor want either one of you to get hurt.  Be direct, clear and honest.  It could be as simple as: "I am looking for a serious committed relationship, and want to make sure you are too, so that I don't waste either your or my time."  If you are discussing being mutually exclusive, tell them that you are ready to make that step because they have such wonderful qualities, and ask if they feel the same about you.

The other person may not be able to give you a yes/no answer right away.  Listen to what they have to say and what their current position is.  Discuss but don't push them to give you an absolute yes/no answer.  You sprang this important conversation topic on them, so give them time to think about it if they need.