Biology .

the living together of two dissimilar organisms, as in mutualism, commensalism, amensalism, or parasitism.

(formerly) mutualism ( def. 1 ) .

Psychiatry . a relationship between two people in which each person is dependent upon and receives reinforcement, whether beneficial or detrimental, from the other.

Psychoanalysis . the relationship between an infant and its mother in which the infant is dependent on the mother both physically and emotionally.

any interdependent or mutually beneficial relationship between two persons, groups, etc.