The term "attachment" refers to a relationship between baby and parent that develops gradually and builds over a long period of time— both parties take a role in the relationship— you could call it a lifelong partnership.
Babies come into the world ready to build relationships with the adults who care for them. Babies communicate with caregivers by gazing at their faces, recognizing their familiar voices, grasping their fingers, smiling at them, and crying when they need or want them.
As babies grow, they develop new ways of communicating and responding to caregivers. If parents learn their baby's cues and provide experiences that the baby finds consistent and responsive to his needs, he will develop a trust in himself and in others--a secure attachment relationship. It takes time for trust to develop, beginning from the earliest interactions between baby and caregiver through the first year of life.
Because this process is one of building a long-term relationship, even infants who did not have immediate contact with their parent (due to adoption, illness, or premature birth) can become securely attached. Even attachment that is not secure at the end of the first year may change for the better if circumstances improve.
Just as relationships between adults are based on what they do together over time, infant/caregiver attachment is also build upon all that is shared over the weeks, months and year of early childhood.