1. A total collection of objects each of which is within some metric distance of at least one other object in the collection. 2. A total collection of objects each of whose image on the focal plane of a sensor is within some metric distance of the image of at least one other object in the collection. 3. A set of objects with similar state vectors (based on truth). For example, a reentry vehicle and its penaids deployed at virtually the same time from a post-boost vehicle. 4. For BM/C3 purposes, a cluster is a group of objects any one of which can be engaged by an interceptor launched at the centroid of the cluster, possibly before the cluster is resolved into separate objects.