A Sigil (シギル) is an ability all players taking part in the Darwin's Game possess, granted once they join the game.
Not much is known about sigils other than they are supernatural and superhuman abilities granted to players of the Darwin's Game. All players who take part in the game are immediately granted a randomized and unique sigil upon creating an account. Players automatically know how to use their sigil and it is impossible to change your sigil. Executives of the Kuchinawakai are all said to have the same sigil.
In the extra chapter of volume 7, Kashiwagi Rein states that there 4 ranks to sigils and lists their percentages along with her own reasoning on the naming rules. 75% are "Animal Rank" sigils that have an animal's name and either transform the body or expand the user's 5 senses, less than 20% are "Superhuman Rank" sigils whose names are based on the function of the sigil and many are unique, less than 4% are "King Rank" sigils that have the kanji of royalty in them and are mostly powerful telekinetic types, and less than 1% are the extremely rare "Myth Class" sigils that are named after gods.
People from Japan in World Line O refer to sigils as Koujinryoku (荒神力) and only officers of the imperial guard have been shown to use it so far. It is stated that the imperial family is apparently monopolizing the koujinryoku and there are those that don't like it as they believe sharing it to everyone would result in equality. Those people are later revealed to be agents of the Great Russian Republic and they refer to sigils as Exen (エクセン).
Telekinetic (念動系) type sigils give the user the power to manipulate specific materials or objects, which can range from the user's own body, other people, or just objects. Examples of this type include Karino Shuka's Queen of Thorn and Wang's Beelzebub.
Metamorphic (身体変化系) type sigils can either completely or partially change's the user's body to a different quality or shape. Examples of this type include Danjou's Wall Iron and Hiiragi Suzune's unknown sigil.
Mind Control (精神操作系) type sigils are able to directly influence the mind of the target. These sigils have the ability impose thoughts, actions, and/or demands upon the subject, though some only work on humans and specific animals. Examples of this type include Ayanokouji Kaito's Peace Maker and Themis' Unbalance.
Sense Expansion (感覚拡張系) type sigils strength either the user's five senses or their sixth sense, and because of this they are also referred to as ESP. Examples of this type include Maesaka Ryuuji's True or Lie and Kashiwagi Rein's Laplace