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 and governments.
The word "shill" is probably related to "shilliber", or alternatively "shillaber", a word of obscure early-20th century origin with the same meaning. One possible explanation for the origin of the word is the English name "Shillibeer". George Shillibeer was an entrepreneur who developed the first commercial bus service in the UK. Shillibeer was believed to employ confederates who could help him solicit more customers for his bus line.
In the UK the term plant is used, being similar with the North American 'shill' in many instances, referring to having someone planted in the audience or in a crowd, or 'to plant drugs' (on someone).
Shilling is illegal in many circumstances and in many jurisdictions 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 appropriate (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, as their income is tied to its prosperity.