Zoology term. (I am not a zoologist.)
Term for the phenomenon in which individuals of a species "mate for life", ie a breeding pair is united for the lifespan of one or both individuals.
Pair bonding is quite prevalent among birds and mammals. Pair bonding is advantageous among species whose offspring have a relatively long immature phase because it splits the responsibilities for parenting between a couple, rather than an individual.
Pair bonding may occur to a greater or lesser extent; it is not a black-or-white trait. For example, one species might pair bond for a single season (many migratory birds do this), while another might do so for life.
Pair bonding also refers to the process of establishing a pair bond (eg courtship rituals, marriage) as well as the ongoing psychological changes needed to adjust to life as part of a team, rather than as a solitary individual.
Humans, in general, do tend to pair bond, though rapid societal changes are occuring. As always with homo sapiens, individuals are free to make their own decisions about how to best live their life; thus opinions on this topic tend to vary widely.