1. Resistance to displacement of a restoration in an occlusal direction; this may be aided by the use of adhesives, cements, or bonding agents. It supplements resistance form. Pins may be used to improve the retention of both direct and indirect restorations.
2. Resistance to displacement of a denture in a direction opposite to the path of insertion. Direct retention of a removable partial denture is obtained by the use of clasps, attachments, or retentive arms on the supporting abutment teeth. Indirect retention ( anti-rotation) of a removable partial denture is obtained by the use of indirect retainers.
3. The period following the completion of orthodontic appliance therapy during which the teeth are stabilized in the desired position with the use of a retainer.
McNally M., Mullin M., Dhopatkar A., Rock W. P. Orthodontic retention: why when and how? Dent Update 2003;30(8):446–52.