Simply speaking tacit consent is consent that is implied or not expressly stated . Tacit consent is one of the main topics of Lockean philosophy.
Stanford Encyclopedia of Philosophy (2012):
Simply by walking along the highways of a country a person gives tacit consent to the government and agrees to obey it while living in its territory. This, Locke thinks, explains why resident aliens have an obligation to obey the laws of the state where they reside, though only while they live there. Inheriting property creates an even stronger bond, since the original owner of the property permanently put the property under the jurisdiction of the commonwealth.
As Bowen Greenwood notes (1995):
Locke writes, “[E]very man, that hath any Possession, or Enjoyment, of any part of the Dominions of any Government, doth thereby give his tacit Consent, and is as far forth obliged to Obedience to the laws of that government, during such enjoyment, as any one under it.”
Tacit consent is an important concept in political philosophy. According to this theory the anti-war agitator that rallies on the public commons has given consent to the government’s war.
Oxford English Dictionary Online
Greenwood, B. (1995). “Tacit Consent: A Quiet Tyranny.” The Freeman. Retrieved from http://fee.org/freeman/detail/tacit-consent-a-quiet-tyranny.
Tuckness, A. (2012). “Locke’s Political Philosophy”, The Stanford Encyclopedia of Philosophy. Retrieved from http://plato.stanford.edu/archives/win2012/entries/locke-political/.