A more thorough explanation:

Definition: A sham marriage in which a U.S. citizen marries a foreign citizen for the sole purpose of allowing the foreign citizen to become a permanent U.S. resident.

Example: John, a U.S. citizen, marries Maria, a foreign citizen, solely for the purpose of helping her obtain a green card. They do not intend to live together as husband and wife and have no real relationship. This is an example of a green-card marriage.

Explanation: A green-card marriage is a fraudulent marriage that is entered into for the purpose of circumventing U.S. immigration laws. The U.S. government has enacted laws to regulate such marriages, and those who engage in them can face serious legal consequences. In the example given, John and Maria's marriage is not a genuine marriage, but rather a scheme to obtain a green card for Maria. This type of marriage is illegal and can result in both civil and criminal penalties.