Window (casing) : An exit point of a lateral from a mother bore, generally a hole cut in the side of the casing to allow sidetracking the well. Can also be where the entire section of the casing is removed
George E. King Engineering, 5000 Oilfield Terms: A Glossary of Petroleum Engineering Terms, Abbreviations and Acronyms, US 2010. http://gekengineering.com/Downloads/Free_Downloads /Glossary_of_Petroleum_Engineering_Terms_25_August _2010.pdf, (Oil - Amer) {593}