Final answer:
Attachment is the affectional bond or tie that forms between an infant and their primary caregiver. It is a major social milestone of infancy and involves a secure base provided by the caregiver. Mary Ainsworth's research identified different types of attachment.
Step-by-step explanation:
Attachment is the affectional bond or tie that an infant forms with the primary caregiver. It is a long-standing connection or bond with others and is a major social milestone of infancy. John Bowlby defined attachment as a secure base where a parent provides a sense of safety as the child explores the environment. Mary Ainsworth's research identified different types of attachments including secure, avoidant, resistant, and disorganized.