DigiCore

The DigiCore is a type of nucleus, and a powerful energy source, that houses a Digimon's primary data, and is analagous to its soul; a Digimon can continue to exist long after its body has rotted away so long as its DigiCore remains unharmed, but this also means that the DigiCore is a point of failure for the Digimon, as the Digimon will immediately cease functioning if they lose their DigiCore. As most Digimon only have one DigiCore, they usually hide it deep within their body, although many Undead Digimon have rotted to such an extent that their DigiCores are visible, and the most powerful of Mega Digimon, such as the Digimon Sovereigns, each have a dozen DigiCores that orbit their bodies. In addition, when two Digimon undergo DNA Digivolution, their DigiCores are normally totally fused, resulting in a single new Digimon; when this process is left incomplete, the resulting Digimon maintains its form in a very unstable state. UltimateKhaosmon, who is both an extremely powerful Mega Digimon and an incomplete DNA Digivolution, is thus unable to store its DigiCores within its body, so they manifest bare.

Digimon Adventure 02
The Digicores have amazing power, as seen when Azulongmon gave Gennai one of his, which in turn allowed for to become  for the first time. It also gave the original DigiDestined's Digimon the power to digivolve to their Ultimate and Mega level once again. Another one of Azulongmon's also allowed to digivolve to. Also, if one of the DigiCores are gone from one of the Four Harmonious Ones, then the Digital World's defenses will weaken, making the Digital World unbalanced.

Digimon Fusion
The Death General can remove his own DigiCore from his body and place it elsewhere for safekeeping.

Digimon Next
The Black DigiCore is an artifact that appears in Digimon Next. uses them to enslave Digimon and humans to his will by inserting them into the target.