According to psychologist Mary Ainsworth, "attachment" may be defined as "an affectional tie that one person or animal forms between himself and another specific one – a tie that binds them together in space and endures over time."
Attachment is not just a connection between two people; it is a bond that involves a desire for regular contact with that person and the experience of distress during separation from that person.
Psychologist John Bowlby is generally thought of as the father of attachment theory. He defined attachment as a "lasting psychological connectedness between human beings." Childhood, he argues, played a critical role in the formation of attachments and early experiences could have an impact on the relationships people form later in life.
The earliest attachments we form are with parents and other caregivers, which is perhaps why Bowlby believed that attachment had a strong evolutionary component. These early attachments with caregivers serve to keep an infant safe and secure and ensures the child's survival.