I often hear people use the term "intimacy" as a euphemism for coitus or sexual activities. I think the first time it sunk in for me was when I noticed for the umpteenth time that Marge Simpson uses the word in this way in many episodes of The Simpsons when she wishes to "get physical" with Homer. After studying and teaching psychology for so many years, I see that this, like "antisocial" is yet another term that needs some clarification.
Intimacy: a psychological and emotional bond (NOT physical!) between two individuals. Said individuals do not necessarily have to be involved in a romantic relationship with one another. They could be blood relatives (siblings, parent/child, etc.) or very good friends. Intimacy means that you can "read" that person's mood just by looking at their facial expressions or body posture. If you can walk into a room and know that this is a good time to share a funny story, or a good time to give the person some space, then you have a high level of intimacy with that individual. The stronger your emotional and psychological bond with a person, the more intimate you are with them. This is separate from sex. While intimacy can make a sexual relationship more meaningful, it really has very little to do with any physical activities you engage in with another person. That is known as passion.
Passion: the emotional and physical arousal a person experiences when in the presence of another person. True, passion can run the gamut from hate (if extreme negative emotions and a desire to cause physical harm to another person occur in that person's presence) to love (if a desire to bring happiness to the person and a great concern for their well-being occurs in the person's presence) to lust (if you experience a physical draw and desire for physical sexual contact with the person). Passion is arousal (used here in the psychological sense). Intimacy is a bond between two individuals. And yes, a person can have a "passion" for a hobby or object. This means that the person experiences extreme emotions when thinking about or confronting said subject.
Arousal: activation of one's interest and increase in stress hormone levels. Arousal can be bad, in that great levels of extreme arousal can lead to too much stress and a person could suffer symptoms of an anxiety disorder. Arousal can also be bad if it comes in too little quantities. Think about how you feel when you are "bored to tears" because you just don't seem to have enough to do or enough motivation to do something. Arousal can also be good. A certain optimal level of arousal (dependent upon the individual and the task at hand) gives a person just enough reason to get up and get moving, while not overwhelming them. Arousal can be mental, emotional, and physical. Physical arousal is not always sexual in nature, either. A person can experience physical arousal when they succeed at a difficult task (think pride and a desire to celebrate) or when they are frustrated or suffer a loss (think about any time you felt angry enough that you wanted to punch something or throw something). Physical arousal occurs when our autonomic nervous system, specifically our sympathetic nervous system, triggers the release of stress hormones (e.g. adrenaline, cortisol, DHEA) so that we can enter our fight-or-flight mode in order to tackle whatever is causing the stress reaction. [I will post a future blog entry on basic stress definitions]. Sometimes this physical activation leads to arousal of the sexual organs, sometimes it does not.
In summation, intimacy is a close emotional and psychological bond, while passion is emotional and physical arousal. If a couple is engaging in sex, they may be increasing their intimacy, but essentially they are operating under a physical passion. Arousal usually means you are interested, though not always physically.