An Invisalign Attachment is a small bump of tooth-colored composite material that is bonded to a tooth with a curing light. The Attachment serves as a knob which helps the aligner grip the tooth more tightly and provide the force to achieve a specific kind of movement. Attachments may be shaped in different ways: rectangular, square, circular or triangular. Sometimes orthodontic elastics are attached to them. After your Invisalign treatment is finished (or when the Attachments are no longer needed), your dentist or orthodontist can easily remove them.
In other aligner systems (such as ClearCorrect), Attachments may have other names, such as Engagers. No matter what they are called, they serve the same sort of purpose.