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).