close familiarity or friendship; closeness.
So often we forget about the friendship and open communication we long for in a relationship. When you are dating someone or are in a partnership with them, they become your best friend, the person you tell your secrets to, the one who is by your bedside in the hospital and who listens to your venting about work. You create a bond with them because they are there for you when you need them, and that is what intimacy is about. It is being as close as possible, vulnerable, open, and willing to trust the other person in your most primal moments.
When you are having sex, that intimacy can come in many forms. It can be the connection you feel when you lock eyes, it can be the spiritual connection you feel when kissing them, or it can be in the moment you both climax together and know that you are in sync in both your sexual life and personal life. Being in sync also means you are on the same level, emotionally, mentally and personally. You can talk to each other and have an in-depth conversation, you can listen and understand their feelings and point of view, you are supportive of their work and friends, their family and their life choices. Things are good.
When some of those parts are off, you cannot be completely in sync and then your intimacy suffers because you are holding back regrets, negative feelings, words you want to say but can't, secrets you haven't revealed, etc. There are closed doors between you and therefore you are not fully trusting that you can have that open communication needed for a fully intimate relationship.
Sex can be good without complete intimacy, but it is lasting, bonding and different, when you can be fully intimate with your partner.