A shill is an associate of a person selling goods or services or a political group, who pretends no association to the seller/group and assumes the air of an enthusiastic customer. The intention of the shill is, using crowd psychology, to encourage others unaware of the set-up to purchase said goods or services or support the political group's ideological claims. Shills are often employed by confidence artists. The term plant is also used.

Shilling is illegal in many circumstances and in many jurisdictions[1] because of the frequently fraudulent and damaging character of their actions. However, if a shill does not place uninformed parties at a risk of loss, but merely generates "buzz", the shill's actions may be legal. For example, a person planted in an audience to laugh and applaud when desired (see claque), or to participate in on-stage activities as a "random member of the audience", is a type of legal shill.

"Shill" can also be used pejoratively to describe a critic who appears either all-too-eager to heap glowing praise upon mediocre offerings, or who acts as an apologist for glaring flaws. In this sense, they would be an implicit "shill" for the industry at large, possibly because their income is tied to its prosperity.

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