cD Galaxies
M87 is a cD galaxy, technically.  Kormendy
(1982) distinguishes these from being merely
giant ellipticals.
Extensive stellar envelope up to 100 kpc
Only in regions of enhanced galaxy density (a
factor of 100 denser than the average)
Multiple nuclei in 25-50% of cDs (a very rare
thing)
Regular cD clusters are systems that have
relaxed into dynamical equilibrium.