Groups of cores and supporting components. E.g. what in a multi-socket system would be different CPUs, each with their own memory and IO interfaces, and interconnects between them, although multiple of these can also live on a single chip.
Although not a hard and fast rule, it is commonplace to use the term clusters for CPUs that share a common cache at some level (typically L2). This is quite prevalent in Arm designs, for example, such as big.LITTLE compositions where a big CPU cluster would share an L2 cache, the LITTLE cluster would share another and system software would set up the cache ability and share ability domains (arm parlance for silos where cache maintenance operations can propagate) to reflect that topology.