Astronomically speaking, the seventh house is that one-twelfth sector of the local sky just above the western horizon (see above picture) occupied by planets in the two hours just before they set.

The first of the social houses, the seventh house is opposed to the first house literally and in emphasis: in the first house we put our energies into the world; in the seventh the world puts energy back into us. The purpose of the seventh house is to bring about greater self-awareness through relating to what we experience as being outside of ourselves. This may in fact be energy within us that we have rejected out of fear or the need to experience it more objectively or externally. This is called projection, and we seek projected parts of ourselves in our intimate relationships.

Thus the seventh house rules intimate one-to-one relationships of all kinds (including marriage, business and counseling relationships), open enemies and conflicts, lawsuits and the lower courts. Planets in the seventh house indicate areas in our lives that need relationships to bring them out, and the houses ruled by seventh house planets are those "areas." Many planets here indicate many needs that we are looking for others to fill. In this case many relationships (or more than one marriage) is probable, as it’s unlikely that one individual would fill all those needs. Individuals with so much projected energy feel a void in their lives, and may encourage others to be dependent, or even pick fights rather than be alone.

This Seventh House of Astrology Houses page and much of this 550-page resource website are taken from You and the Universe.