A union between one man and one woman.
"And the LORD God said, 'It is not good that man should be alone; I will make him a helper comparable to him.' Out of the ground the LORD God formed every beast of the field and every bird of the air, and brought them to Adam to see what he would call them. And whatever Adam called each living creature, that was its name. So Adam gave names to all cattle, to the birds of the air, and to every beast of the field. But for Adam there was not found a helper comparable to him. And the LORD God caused a deep sleep to fall on Adam, and he slept; and He took one of his ribs, and closed up the flesh in its place. Then the rib which the LORD God had taken from man He made into a woman, and He brought her to the man. And Adam said: 'This is now bone of my bones And flesh of my flesh; She shall be called Woman, Because she was taken out of Man.' Therefore a man shall leave his father and mother and be joined to his wife, and they shall become one flesh. And they were both naked, the man and his wife, and were not ashamed." Genesis 2:18-25