Two or more characters (frequently twins) are in such perfect harmony that they seem almost to be one person with two bodies. They finish each other's sentences, never seem to need to talk to communicate, and may even know what is happening to each other from far away.
In Sci-Fi or Fantasy series, the connection may actually be a true shared mind, either with each member contributing to the whole, or the separate bodies being puppets which some central mind controls remotely. It makes sense that usually the first variant is sensitive to losses and avoids overt violence, but in the second, it's only the question of whether lost bodies will be replenished and it's inclined to expand itself. Expect "individuals" in such hives to be considered very killable by everyone else as well. It is not unusual for it to start out as the former and then slip into the latter as a series progresses and the writing staff changes. The Virus is often a Hive Mind (e.g., the Borg) and the Evil Matriarch becomes its Hive Queen.
There is a traditional tendency in SF and fantasy shows for Hive Mind species to be xenophobic, aggressive, and evil, even when they aren't a Horde of Alien Locusts. This may well be due to a perceived metaphorical overlap with Dirty Communists (even when a Hive Queen is present, which would technically make them a caste system), or a residual, primal fear relating to eusocial insects, which are the closest thing to Hive Minds in Real Life. It could be justified, however, as it's possible that a truly hive-minded species may literally never encounter another sentient entity until it achieves interstellar travel, and so have a distinct lack of social skills. This trope is particularly common among transhumanist works, however, where an advanced level of technology is assumed.
Hive minds are not known to exist in reality — hive insects, upon which the idea is based, communicate intentions and commands through scent and body language. The closest approach to them would be the controversial superorganism concept. However, it should be noted that a somewhat rough form of this does occur in humans presently: the advent of the internet in the early 90's not only enabled the instantaneous transfer of global information and knowledge to the point that many people, even swaths of strangers are able to communicate and share their information rather rapidly in modern society and you'd be hard-pressed to find a corner of the Earth connected via the Web that would ignore another group or individual's suffering upon it being posted online, even if that person(s) isn't even remotely or tangentially related to them.
Related to, but separate, from Synchronization, where each individual experiences what the other does without necessarily being in rapport with each other. Contrast both Mind Hive and Many Spirits Inside of One, both its complete opposites, when multiple minds or Split Personalities are sharing one body (differentiated by the level of accord between them and/or their "host"); and Pieces of God, where a Cosmic Entity is split into several pieces, which may or may not be living entities themselves. May be controlled by a Hive Queen, which serves as the titular keystone of a Keystone Army of Hive Drones. See also Psychic Link for other connections between minds. A related plot is The Evils of Free Will. Compare Split-Personality Merge, where two or more Split Personalities become one. When the person speaks in plural, see I Am Legion. If the hive mind is controlling many smaller creatures that forms the shape of a larger creature, you may be dealing with The Worm That Walks.
See Mental Fusion for the comparatively less identity-diluting and usually temporary version.
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