Dyad: The word "dyad" comes from the Greek "dyas" meaning the number two.
In psychology, a dyad refers to a pair of persons in an interactional situation. For example, a patient and therapist, a woman and her husband, a girl and her stepfather, etc.
In chemistry, a dyad is a bivalent element. And in biology, a dyad is a double chromosome resulting from the splitting of a tetrad (a quadruple chromosome) during meiosis (germ cell formation).