A cornfield was a tract of land, specifically a field, devoted to the growing of corn.
In April 2151, a one-man Klingon K'toch-class scout ship crash landed in a cornfield in Broken Bow, Oklahoma. (ENT: "Broken Bow")
In July 2151, when discussing where to land on the Akaali homeworld, T'Pol suggested a remote farm to reduce risk of cultural contamination; Archer jokingly replied that had to be the reason why aliens always landed in cornfields. (ENT: "Civilization")